RETIREMENT PLAN SHARES

These securities have not been approved or disapproved by the Securities and Exchange Commission nor has the Securities and Exchange Commission passed upon the accuracy or adequacy of this Prospectus. Any representation to the contrary is a criminal offense.

Beginning on January 1, 2021, as permitted by regulations adopted by the Securities and Exchange Commission, paper copies of the Fund’s shareholder reports will no longer be sent by mail, unless you specifically request paper copies of the reports from the Fund or from your financial intermediary, such as a broker-dealer or bank. Instead, the reports will be made available on a website, and you will be notified by mail each time a report is posted and provided with a website link to access the report.

If you already elected to receive shareholder reports electronically, you will not be affected by this change and you need not take any action. You may elect to receive shareholder reports and other communications from the Fund or your financial intermediary electronically by going to thornburg.com/edelivery.

You may elect to receive all future reports in paper free of charge. You can inform the Fund or your financial intermediary that you wish to continue receiving paper copies of your shareholder reports by calling 800.847.0200 or your financial intermediary directly. Your election to receive reports in paper will apply to all funds held with the fund complex/your financial intermediary.

Thornburg investment trust

Funds Prospectus

FEBRUARY 1, 2020

Thornburg investment trust

Table of Contents

Fund Summary

Limited Term U.S. Government Fund

Investment Goal

The primary goal of Limited Term U.S. Government Fund is to provide as high a level of current income as is consistent, in the view of the Fund’s investment advisor, with safety of capital. As a secondary goal, the Fund seeks to reduce changes in its share price compared to longer term portfolios.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, and Class R5 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 0.99%, 0.99%, and 0.67%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary Limited Term U.S. Government Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 29.37% of the average value of its portfolio.

Principal Investment Strategies

Thornburg Investment Management, Inc. (“Thornburg”) actively manages the Fund’s portfolio in pursuing the Fund’s investment goals. While Thornburg follows domestic and international economic developments, outlooks for securities markets, interest rates and inflation, the supply and demand for debt obligations, and other factors, the Fund’s investments are determined by individual security analysis. The Fund ordinarily acquires and holds securities for investment rather than for realization of gains by short-term trading on market fluctuations. However, it may dispose of any security before its scheduled maturity to enhance income or reduce loss, to change the portfolio’s average maturity, or to otherwise respond to market conditions.

Limited Term U.S. Government Fund invests at least 80% of its assets in U.S. Government Securities. For this purpose, “U.S. Government Securities” means:

Securities backed by the full faith and credit of the U.S. government, including direct obligations of the U.S. Treasury (such as U.S. Treasury Bonds) and obligations of U.S. government agencies and instrumentalities which are guaranteed by the U.S. Treasury (such as “Ginnie Mae” mortgage-backed certificates issued by the Government National Mortgage Association).

Securities issued or guaranteed by U.S. government agencies, instrumentalities or sponsored enterprises, but which are not backed by the full faith and credit of the U.S. government. These securities include mortgage-backed certificates, collateralized mortgage obligations (“CMOs”), and debentures issued by “Freddie Mac” (Federal Home Loan Mortgage Corporation) and “Fannie Mae” (Federal National Mortgage Association).

U.S. Government Securities include for this purpose repurchase agreements secured by the securities described above, and participations having economic characteristics similar to those securities. “Participations” are undivided interests in pools of securities where the underlying credit support passes through to the participants.

Because the magnitude of changes in the value of interest-bearing obligations is greater for obligations with longer terms given an equivalent change in interest rates, the Fund seeks to reduce changes in its share value by maintaining a portfolio of investments with a dollar-weighted average maturity or expected life of normally less than five years. There is no limitation on the maturity of any specific security the Fund may purchase, and the Fund may sell any security before it matures. The Fund also attempts to reduce changes in share value through credit analysis, selection and diversification.

Fund Summary Limited Term U.S. Government Fund

Principal Investment Risks

Although the Fund may acquire obligations issued or guaranteed by the U.S. government and its agencies, instrumentalities and enterprises, neither the Fund’s net asset value nor its dividends are guaranteed by the U.S. government. An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares and its dividends may fluctuate from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. If your sole objective is preservation of capital, then the Fund may not be suitable for you because the Fund’s share value will fluctuate, including as interest rates change. Investors whose sole objective is preservation of capital may wish to consider a high quality money market fund. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term obligations. This effect is also typically more pronounced for the Fund’s investment in mortgage-backed securities, the value of which may fluctuate more significantly in response to interest rate changes. When interest rates decrease, the Fund’s dividends may decline.

Prepayment and Extension Risk – When market interest rates decline, certain debt obligations held by the Fund may be repaid more quickly than anticipated, requiring the Fund to reinvest the proceeds of those repayments in obligations which bear a lower interest rate. Conversely, when market interest rates increase, certain debt obligations held by the Fund may be repaid more slowly than anticipated, causing assets of the Fund to remain invested in relatively lower yielding obligations. These risks may be more pronounced for the Fund’s investments in mortgage-backed securities.

Credit Risk – All securities owned by the Fund may be subject to default, delays in payment, adverse legislation or other government action, or could be downgraded by ratings agencies, reducing the value of the Fund’s shares. Securities backed by the full faith and credit of the U.S. government, such as U.S. Treasury obligations, are commonly regarded as having small exposure to credit risk. Obligations of certain U.S. government agencies, instrumentalities and government-sponsored enterprises (sometimes referred to as “agency obligations”) are not direct obligations of the United States, may not be backed by the full faith and credit of the U.S. government, and may have a greater exposure to credit risk. Although the U.S. government is required by law to provide credit support for some agency obligations, there is no assurance that the U.S. government would provide financial support for any such obligation on a default by the issuing agency, instrumentality or enterprise in the absence of a legal requirement to do so. As of the date of this Prospectus, securities backed by the full faith and credit of the U.S. government, and securities of U.S. government agencies, instrumentalities and enterprises which may be purchased by the Fund are rated “Aaa” by Moody’s Investors Services or “AA+” by S&P Global Ratings. Ratings agencies may reduce the ratings of any securities in the future.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. For example, a fall in worldwide demand for U.S. government securities or general economic decline could lower the value of those securities.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell promptly some or all of the obligations that it holds, or may only be able to sell obligations at less than desired prices.

Structured Products Risk – Investments in securities that are backed by, or represent interests in, an underlying pool of securities or other assets involve the risks associated with the underlying assets (e.g., the risk of default by mortgagors), and may also involve different or greater risks, including the risk that distributions from the underlying assets will be inadequate to make interest or other payments to the Fund, the risk that the issuer of the securities will fail to administer the underlying assets properly or become insolvent, and the risk that the securities will be less liquid than other Fund investments.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61.

Fund Summary Limited Term U.S. Government Fund

Past Performance of the Fund

The following information provides some indication of the risks of investing in Limited Term U.S. Government Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares have been different in each full year shown. The average annual total return figures compare Class R3, Class R4, and Class R5 share performance to the Bloomberg Barclays US Government Intermediate Total Return Index Value Unhedged, a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Average Annual Total Returns

(periods ended 12-31-19)

Fund Summary Limited Term U.S. Government Fund

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Jason Brady, cfa, the president of the Trust and the chief executive officer, president, and a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2007.

Lon Erickson, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Jeff Klingelhofer, cfa, a managing director and head of investments of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Sales of the Fund’s Class R3 shares are closed to new investors except for certain accounts referenced under the caption “Purchasing Fund Shares,” beginning on page 77 of this Prospectus.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

Limited Term Income Fund

Investment Goal

The primary goal of Limited Term Income Fund is to provide as high a level of current income as is consistent, in the view of the Fund’s investment advisor, with safety of capital. As a secondary goal, the Fund seeks to reduce changes in its share prices compared to longer term portfolios.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, Class R5, and Class R6 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 0.99%, 0.99%, 0.49%, and 0.42%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary Limited Term Income Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 43.22% of the average value of its portfolio.

Principal Investment Strategies

Thornburg Investment Management, Inc. (“Thornburg”) actively manages the Fund’s portfolio in pursuing the Fund’s investment goals. While Thornburg follows domestic and international economic developments, outlooks for securities markets, interest rates and inflation, the supply and demand for debt obligations, and other factors, the Fund’s investments are determined by individual security analysis. The Fund ordinarily acquires and holds securities for investment rather than for realization of gains by short-term trading on market fluctuations. However, it may dispose of any security prior to its scheduled maturity to enhance income or reduce loss, to change the portfolio’s average maturity, or to otherwise respond to current market conditions.

The Fund invests at least 65% of its net assets in (i) obligations of the U.S. government, its agencies and instrumentalities, and (ii) debt obligations rated at the time of purchase in one of the three highest principal long term rating categories of S&P Global Ratings (AAA, AA or A) or Moody’s Investors Services, Inc. (Aaa, Aa or A), or the equivalent three highest short term ratings of those ratings agencies, or, if no credit rating is available, judged to be of comparable quality as determined by Thornburg. The Fund will not invest in any debt obligation rated at the time of purchase lower than BBB by S&P, Baa by Moody’s, the of equivalent short term ratings of those ratings agencies, or, if no credit rating is available, judged to be of comparable quality as determined by Thornburg. The Fund may purchase debt obligations such as corporate debt obligations, mortgage-backed securities, other asset-backed securities, municipal securities, and commercial paper and bankers’ acceptances. The Fund may purchase foreign securities of the same types and quality as the domestic securities it purchases when Thornburg believes these investments are consistent with the Fund’s objectives.

Because the magnitude of changes in the value of interest-bearing obligations is greater for obligations with longer terms given an equivalent change in interest rates, the Fund seeks to reduce changes in its share value by maintaining a portfolio of investments with a dollar-weighted average maturity or expected life normally of less than five years. As a result, the Fund also maintains a portfolio of investments having a dollar-weighted average effective duration of normally no more than five years. Duration is a measure of estimated sensitivity to interest rate changes. A portfolio with a longer average effective duration will typically be more sensitive to interest rate changes than a portfolio with a shorter average effective duration. Duration is commonly expressed as a number, which is the expected percentage change in an obligation’s price upon a 1% change in interest rates. For example, an obligation with a duration of 5 would be expected to change in price by approximately 5% in response to a 1% change in interest rates. During temporary periods the Fund’s portfolio maturity and average effective duration may be reduced for defensive purposes. There is no limitation on the maturity of any specific security the Fund may purchase, and the Fund may sell any security before it matures. The Fund also attempts to reduce changes in share value through credit analysis, selection and diversification.

Fund Summary Limited Term Income Fund

Principal Investment Risks

Although the Fund may acquire obligations issued or guaranteed by the U.S. government and its agencies, instrumentalities and enterprises, neither the Fund’s net asset value nor its dividends are guaranteed by the U.S. government. An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares and its dividends may fluctuate from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. If your sole objective is preservation of capital, then the Fund may not be suitable for you because the Fund’s share value will fluctuate, including as interest rates change. Investors whose sole objective is preservation of capital may wish to consider a high quality money market fund. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term obligations. This effect is also typically more pronounced for the Fund’s investment in mortgage- and other asset-backed securities, the value of which may fluctuate more significantly in response to interest rate changes. When interest rates decrease, the Fund’s dividends may decline.

Prepayment and Extension Risk – When market interest rates decline, certain debt obligations held by the Fund may be repaid more quickly than anticipated, requiring the Fund to reinvest the proceeds of those repayments in obligations which bear a lower interest rate. Conversely, when market interest rates increase, certain debt obligations held by the Fund may be repaid more slowly than anticipated, causing assets of the Fund to remain invested in relatively lower yielding obligations. These risks may be more pronounced for the Fund’s investments in mortgage-backed and asset-backed securities.

Credit Risk – All securities owned by the Fund may be subject to default, delays in payment, adverse legislation or other government action, or could be downgraded by ratings agencies, reducing the value of the Fund’s shares. Securities backed by the full faith and credit of the U.S. government, such as U.S. Treasury obligations, are commonly regarded as having small exposure to credit risk. Obligations of certain U.S. government agencies, instrumentalities and government sponsored enterprises (sometimes referred to as “agency obligations”) are not direct obligations of the United States, may not be backed by the full faith and credit of the U.S. government, and may have a greater exposure to credit risk. Although the U.S. government is required by law to provide credit support for some agency obligations, there is no assurance that the U.S. government would provide financial support for any such obligation on a default by the issuing agency, instrumentality or enterprise in the absence of a legal requirement to do so. As of the date of this Prospectus, securities backed by the full faith and credit of the U.S. government, and securities of U.S. government agencies, instrumentalities and enterprises which may be purchased by the Fund, are rated “Aaa” by Moody’s Investors Services or “AA+” by S&P Global Ratings. Ratings agencies may reduce the ratings of any securities in the future. Lower-rated or unrated obligations in which the Fund is permitted to invest may have, or may be perceived to have, greater risk of default and ratings downgrades.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. For example, a fall in worldwide demand for U.S. government securities or general economic decline could lower the value of those securities.

Risks Affecting Specific Issuers – The value of a debt obligation may decline in response to developments affecting the specific issuer of the obligation, even if other issuers or the overall economy are unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Foreign Investment Risk – Investments in the debt obligations of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection. These risks may be more pronounced for investments in developing countries. In addition, some foreign government debt obligations may be subject to default, delays in payment, adverse legislation or government action, or could be downgraded by ratings agencies.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell promptly some or all of the obligations that it holds, or may only be able to sell obligations at less than desired prices.

Fund Summary Limited Term Income Fund

Structured Products Risk – Investments in securities that are backed by, or represent interests in, an underlying pool of securities or other assets involve the risks associated with the underlying assets (e.g., the risk of default by mortgagors), and may also involve different or greater risks, including the risk that distributions from the underlying assets will be inadequate to make interest or other payments to the Fund, the risk that the issuer of the securities will fail to administer the underlying assets properly or become insolvent, and the risk that the securities will be less liquid than other Fund investments.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61.

Past Performance of the Fund

The following information provides some indication of the risks of investing in Limited Term Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares have been different in each full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to the Bloomberg Barclays Intermediate US Government/Credit Total Return Index Value Unhedged, a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Average Annual Total Returns

(periods ended 12-31-19)

Fund Summary Limited Term Income Fund

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Jason Brady, cfa, the president of the Trust and the chief executive officer, president, and a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2007.

Lon Erickson, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2010.

Jeff Klingelhofer, cfa, a managing director and head of investments of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Fund Summary Limited Term Income Fund

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

Strategic Income Fund

Investment Goal

The Fund’s primary investment goal is to seek a high level of current income. The Fund’s secondary investment goal is some long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, Class R5 and Class R6 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.25%, 1.25%, 0.60%, and 0.53%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary Strategic Income Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 31.55% of the average value of its portfolio.

Principal Investment Strategies

The Fund pursues its investment goals by investing in a broad range of income-producing investments from throughout the world, primarily including debt obligations. The Fund expects, under normal conditions, to invest a majority of its assets in the debt obligations described below.

The Fund may invest in debt obligations of any kind, of any quality, and of any maturity. The Fund expects, under normal conditions, to select a majority of its investments from among the following types of debt obligations:

•bonds and other debt obligations issued by domestic and foreign companies of any size (including lower-rated “high yield” or “junk” bonds)

•mortgage-backed securities and other asset-backed securities, including commercial mortgage-backed securities

•convertible debt obligations

•obligations issued by foreign governments (including developing countries)

•collateralized mortgage obligations (“CMOs”), collateralized debt obligations (“CDOs”), collateralized bond obligations (“CBOs”), and collateralized loan obligations (“CLOs”)

•obligations of the U.S. government and its agencies and sponsored enterprises

•structured notes

•zero coupon bonds and “stripped” securities

•taxable municipal obligations and participations in municipal obligations

The Fund’s investments are determined by individual issuer and industry analysis. Investment decisions are based on domestic and international economic developments, outlooks for securities markets, interest rates and inflation, the supply and demand for debt securities, and analysis of specific issuers. The Fund ordinarily acquires and holds debt obligations for investment rather than for realization of gains by short-term trading on market fluctuations. However, the Fund may dispose of any such investment prior to its scheduled maturity to enhance income or reduce loss, to change the portfolio’s average maturity, or otherwise to respond to market conditions.

Fund Summary Strategic Income Fund

The Fund also may invest in derivative instruments to the extent Thornburg believes such investments may assist the Fund in pursuing its investment goal. Derivatives are financial instruments that derive their value from an underlying asset, reference rate, or index. The Fund may invest in derivatives for risk management purposes, including to hedge against a decline in the value of certain investments. The Fund may also invest in derivatives for non-hedging purposes, including to obtain investment exposures to a particular asset class. Examples of the types of derivatives in which the Fund may invest are options, futures contracts, options on futures contracts, and swap agreements (including, but not limited to, credit default swap agreements).

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares and its dividends may fluctuate from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund. Please note that because the Fund’s objective is to provide high current income, the Fund invests with an emphasis on income, rather than stability of net asset value.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term obligations. This effect is also typically more pronounced for mortgage- and other asset-backed securities, the value of which may fluctuate more significantly in response to interest rate changes. When interest rates decrease, the Fund’s dividends may decline.

Prepayment and Extension Risk – When market interest rates decline, certain debt obligations held by the Fund may be repaid more quickly than anticipated, requiring the Fund to reinvest the proceeds of those repayments in obligations which bear a lower interest rate. Conversely, when market interest rates increase, certain debt obligations held by the Fund may be repaid more slowly than anticipated, causing assets of the Fund to remain invested in relatively lower yielding obligations. These risks may be more pronounced for the Fund’s investments in mortgage-backed and asset-backed securities.

Credit Risk – If obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those obligations may decline and the Fund’s share value and the dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated obligation to pay principal and interest when due is typically less certain than for an issuer of a higher-rated obligation, lower-rated and unrated obligations are generally more vulnerable than higher-rated obligations to default, to ratings downgrades, and to liquidity risk. Debt obligations backed by so-called “subprime” mortgages may also be subject to a greater risk of default or downgrade. Debt obligations issued by the U.S. government or its agencies, instrumentalities and government sponsored enterprises are also subject to credit risk. Securities backed by the full faith and credit of the U.S. government, such as U.S. Treasury obligations, are commonly regarded as having small exposure to credit risk. Obligations of certain U.S. agencies, instrumentalities and enterprises (sometimes referred to as “agency obligations”) are not direct obligations of the U.S. government, may not be backed by the full faith and credit of the U.S. government, and may have a greater exposure to credit risk.

High Yield Risk – Debt obligations that are rated below investment grade and unrated obligations of similar credit quality (commonly referred to as “junk” or “high yield” bonds) may have a substantial risk of loss. These obligations are generally considered to be speculative with respect to the issuer’s ability to pay interest and principal when due. These obligations may be subject to greater price volatility than investment grade obligations, and their prices may decline significantly in periods of general economic difficulty or in response to adverse publicity, changes in investor perceptions or other factors. These obligations may also be subject to greater liquidity risk.

Fund Summary Strategic Income Fund

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. This effect is typically more pronounced for lower-rated and unrated debt obligations (including particularly “junk” or “high yield” bonds), the value of which may fluctuate more significantly in response to poor economic growth or other changes in market conditions, political, economic and legal developments. The market value of any zero coupon bonds or “stripped” securities that the Fund may purchase will typically be more volatile than the value of a comparable, interest-paying bond. Additionally, zero coupon bonds and “stripped” securities are subject to the risk that the Fund may have to recognize income on its investment and make distributions to shareholders before it has received any cash payments on its investment.

Risks Affecting Specific Issuers – The value of a debt obligation may decline in response to developments affecting the specific issuer of the obligation or security, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices. The market for lower-rated and unrated debt obligations (including particularly “junk” or “high yield” bonds) and debt obligations backed by so-called “subprime” mortgages may be less liquid than the market for other obligations, making it difficult for the Fund to value its investment in a lower-rated or unrated obligation or to sell the investment in a timely manner or at an acceptable price.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies may involve additional risks, which may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Foreign Investment Risk – Investments in the debt obligations of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection. In addition, some foreign government debt obligations may be subject to default, delays in payment, adverse legislation or government action, or could be downgraded by ratings agencies.

Developing Country Risk – The risks which may affect investments in foreign issuers (see “Foreign Investment Risk,” above) may be more pronounced for investments in developing countries because the economies of those countries are usually less diversified, communications, transportation and economic infrastructures are less developed, and developing countries ordinarily have less established legal, political, business and social frameworks. At times the prices of debt obligations of a developing country issuer may be extremely volatile. An issuer domiciled in a developed country may be similarly affected by these developing country risks to the extent that the issuer conducts its business in developing countries.

Structured Products Risk – Investments in securities that are backed by, or represent interests in, an underlying pool of securities or other assets involve the risks associated with the underlying assets (e.g., the risk of default by mortgagors), and may also involve different or greater risks, including the risk that distributions from the underlying assets will be inadequate to make interest or other payments to the Fund, the risk that the issuer of the securities will fail to administer the underlying assets properly or become insolvent, and the risk that the securities will be less liquid than other Fund investments.

Derivatives Risk – The Fund’s investments in derivatives involve the risks associated with the securities or other assets underlying the derivatives, and also may involve risks different or greater than the risks affecting the underlying assets, including the inability or unwillingness of the other party to a derivative to perform its obligations to the Fund, the Fund’s inability or delays in selling or closing positions in derivatives, and difficulties in valuing derivatives.

Additional information about Fund investments, investment strategies and risks of investing in the Fund appears below beginning on page 61.

Fund Summary Strategic Income Fund

Past Performance of the Fund

The following information provides some indication of the risks of investing in Strategic Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows the annual total return for Class R3 shares in the one full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to the Bloomberg Barclays US Universal Total Return Index Value Unhedged, a broad measure of market performance, and to a Blended Benchmark comprised of 80% Bloomberg Barclays US Aggregate Bond Total Return Value USD, which represents a broad measure of bond market performance, and 20% MSCI World Net Total Return USD Index, which represents a broad measure of equity market performance in developed markets. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Average Annual Total Returns

(periods ended 12-31-19)

Fund Summary Strategic Income Fund

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Jason Brady, cfa, the president of the Trust and the chief executive officer, president, and a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2007.

Lon Erickson, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Christian Hoffmann, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2018.

Jeff Klingelhofer, cfa, a managing director and head of investments of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Fund Summary Strategic Income Fund

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Sales of the Fund’s Class R3 shares are closed to new investors except for certain accounts referenced under the caption “Purchasing Fund Shares,” beginning on page 77 of this Prospectus.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

Value Fund

Investment Goal

The Fund seeks long-term capital appreciation by investing in equity and debt securities of all types. The secondary, non-fundamental goal of the Fund is to seek some current income.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4 and Class R5 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.35%, 1.25%, and 0.99%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary Value Fund

Example. This Example is intended to help you compare the cost of investing in Value Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 24.94% of the average value of its portfolio.

Principal Investment Strategies

The Fund expects to invest primarily in domestic equity securities (primarily common stocks) selected on a value basis. However, the Fund may own a variety of securities, including foreign equity securities and foreign and domestic debt obligations which, in the opinion of the Fund’s investment advisor, Thornburg Investment Management, Inc. (“Thornburg”), offer prospects for meeting the Fund’s investment goals.

Thornburg intends to invest on an opportunistic basis where the Fund’s portfolio managers believe intrinsic value is not recognized by the marketplace. The Fund seeks to identify value in a broad or different context by investing in a diversified portfolio of stocks the Fund categorizes as basic values, consistent earners, and emerging franchises, when the portfolio managers believe these issues are value priced. The relative proportions of securities invested in each of those categories will vary over time. The Fund seeks to invest in promising companies, and may invest in stocks that reflect unfavorable market perceptions of the company or industry fundamentals. The Fund may invest in companies of any size, but invests primarily in the large and middle capitalization range of publicly traded companies.

Thornburg primarily uses individual issuer and industry analysis to make investment decisions. Value, for purposes of the Fund’s selection criteria, may consider both current and projected measures. Among the specific factors considered by Thornburg in identifying securities for inclusion in the Fund are:

•profitability

•price/earnings ratio

•price/book value ratio

•price/cash flow ratio

•debt/capital ratio

•dividend characteristics

•security and consistency of revenues

•EV (enterprise value)/EBITDA (earnings before interest, taxes, depreciation and amortization) ratio

•undervalued assets

•earnings growth potential

•industry growth characteristics

•industry leadership

•franchise value

•potential for favorable developments

•EBIT (earnings before interest and taxes)/interest expense ratio

Fund Summary Value Fund

The Fund categorizes its equity investments in the following three categories:

Basic Value: Companies generally operating in mature industries and which generally exhibit more economic sensitivity and/or higher volatility in earnings and cash flow.

Consistent Earner: Companies which generally exhibit predictable growth, profitability, cash flow and/or dividends.

Emerging Franchise: Companies with the potential to grow at an above average rate because of a product or service that is establishing a new market and/or taking share from existing participants.

Inclusion of any investment in any of the three described categories represents the opinion of the advisor concerning the characteristics and prospects of the investment. There is no assurance that any company selected for investment will, once categorized in one of the three described investment categories, continue to have the positive characteristics or fulfill the expectations that the advisor had for the company when it was selected for investment, and any such company may not grow or may decline in earnings and size.

The Fund selects foreign securities issued by companies domiciled in countries whose currencies are freely convertible into U.S. dollars, or in companies in other countries whose business is conducted primarily in U.S. dollars (which could include developing countries).

Debt obligations may be considered for investment if Thornburg believes them to be more attractive than equity alternatives, or to manage risk. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase.

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies may involve additional risks, which may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection. These risks may be more pronounced for investments in developing countries.

Fund Summary Value Fund

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those debt obligations may decline and the Fund’s share value and any dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated debt obligation (including particularly “junk” or “high yield” bonds) to pay principal and interest when due is typically less certain than for an issuer of a higher-rated debt obligation, lower-rated and unrated debt obligations are generally more vulnerable than higher-rated debt obligations to default, to ratings downgrades, and to liquidity risk.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term debt obligations. Decreases in market interest rates may result in prepayments of debt obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61.

Past Performance of the Fund

The following information provides some indication of the risks of investing in Value Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares have been different in each full year shown. The average annual total return figures compare Class R3, Class R4 and Class R5 share performance to the S&P 500 Total Return Index, a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Fund Summary Value Fund

Average Annual Total Returns

(periods ended 12-31-19)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

The performance information shown above may include gains attributable to the Fund’s investments in shares of companies through initial public offerings (“IPOs”). There can be no assurance that the Fund will have continued access to profitable IPOs and, as the Fund’s assets grow, the impact of the Fund’s investment in IPOs on the performance of the Fund may decline.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Manager:

Robert MacDonald, cfa, a managing director of Thornburg, has been a portfolio manager of the Fund since 2015.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Fund Summary Value Fund

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

International Value Fund

Investment Goal

International Value Fund seeks long-term capital appreciation by investing in equity and debt securities of all types. The secondary, non-fundamental goal of the Fund is to seek some current income.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, and Class R5 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.45%, 1.25%, and 0.99%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary International Value Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 71.09% of the average value of its portfolio.

Principal Investment Strategies

The Fund invests primarily in foreign equity securities or depository receipts of foreign equity securities. The Fund may invest in developing countries, but under normal conditions those investments are expected to comprise a significantly smaller proportion of the Fund than investments in developed countries.

The Fund’s investment advisor, Thornburg Investment Management, Inc. (“Thornburg”), intends to invest on an opportunistic basis where the Fund’s portfolio managers believe intrinsic value is not recognized by the marketplace. The Fund seeks to identify value in a broad or different context by investing in a diversified portfolio of stocks the Fund categorizes as basic values, consistent earners, and emerging franchises, when the portfolio managers believe these issues are value priced. The relative proportions of securities invested in each of those categories will vary over time. The Fund seeks to invest in promising companies, and may invest in stocks that reflect unfavorable market perceptions of the company or industry fundamentals. The Fund may invest in companies of any size, but invests primarily in the large and middle capitalization range of publicly traded companies.

Thornburg primarily uses individual issuer and industry analysis to make investment decisions. Value, for purposes of the Fund’s selection criteria, may consider both current and projected measures. Among the specific factors considered by Thornburg in identifying securities for inclusion in the Fund are:

•profitability

•price/earnings ratio

•price/book value ratio

•price/cash flow ratio

•debt/capital ratio

•dividend characteristics

•security and consistency of revenues

•EV (enterprise value)/EBITDA (earnings before interest, taxes, depreciation and amortization) ratio

•undervalued assets

•earnings growth potential

•industry growth characteristics

•industry leadership

•franchise value

•potential for favorable developments

•EBIT (earnings before interest and taxes)/interest expense ratio

Fund Summary International Value Fund

The Fund categorizes its equity investments in the following three categories:

Basic Value: Companies generally operating in mature industries and which generally exhibit more economic sensitivity and/or higher volatility in earnings and cash flow.

Consistent Earner: Companies which generally exhibit predictable growth, profitability, cash flow and/or dividends.

Emerging Franchise: Companies with the potential to grow at an above average rate because of a product or service that is establishing a new market and/or taking share from existing participants.

Inclusion of any investment in any of the three described categories represents the opinion of the advisor concerning the characteristics and prospects of the investment. There is no assurance that any company selected for investment will, once categorized in one of the three described investment categories, continue to have the positive characteristics or fulfill the expectations that the advisor had for the company when it was selected for investment, and any such company may not grow or may decline in earnings and size.

Debt obligations may be considered for investment if Thornburg believes them to be more attractive than equity alternatives, or to manage risk. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase.

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection.

Developing Country Risk – The risks which may affect investments in foreign issuers (see “Foreign Investment Risk,” above) may be more pronounced for investments in developing countries because the economies of those countries are usually less diversified, communications, transportation and economic infrastructures are less developed, and developing countries ordinarily have less established legal, political, business and social frameworks. At times the prices of equity securities or debt obligations of a developing country issuer may be extremely volatile. An issuer domiciled in a developed country may be similarly affected by these developing country risks to the extent that the issuer conducts its business in developing countries.

Risks Affecting Specific Countries or Regions – If a significant portion of the Fund’s assets is invested in issuers that are economically exposed to one country or region, the Fund’s share value may be more susceptible to the conditions and developments in that country or region, and potentially more volatile than the share value of a more geographically diversified fund. The nature and degree of the risks affecting a given country or region, and the extent of the Fund’s exposure to any such country or region, is expected to vary over time.

Fund Summary International Value Fund

Risks Affecting Investments in China – In recent years, a significant portion of the Fund’s assets has been invested in companies exposed to China. Risks affecting investments in China include a potential downturn in the Chinese economy, reduced liquidity or increased price volatility in the Chinese securities markets as a result of trading suspensions affecting Chinese issuers or other factors, the potential that the Chinese government may expand restrictions on foreign investments or the repatriation of capital, and the implementation of new tariffs or other trade barriers by China or its trading partners.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies may involve additional risks, which may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those debt obligations may decline and the Fund’s share value and any dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated debt obligation to pay principal and interest when due is typically less certain than for an issuer of a higher-rated debt obligation, lower-rated and unrated debt obligations are generally more vulnerable than higher-rated debt obligations to default, to ratings downgrades, and to liquidity risk.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term debt obligations. Decreases in market interest rates may result in prepayments of debt obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61 of the Prospectus.

Past Performance of the Fund

The following information provides some indication of the risks of investing in International Value Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares vary in each full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to the MSCI EAFE Net Total Return USD Index and the MSCI ACWI ex USA Net Total Return USD Index, each of which is a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Fund Summary International Value Fund

Average Annual Total Returns

(periods ended 12-31-19)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Fund Summary International Value Fund

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Lei Wang, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2006.

Di Zhou, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2015.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

Growth Fund

Investment Goal

The Fund seeks long-term growth of capital by investing in equity securities selected for their growth potential.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4 and Class R5 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.50%, 1.40%, and 0.99%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary Growth Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 40.69% of the average value of its portfolio.

Principal Investment Strategies

The Fund expects to invest primarily in domestic equity securities (primarily common stocks) selected for their growth potential. However, the Fund may own a variety of securities, including foreign equity securities. The Fund may invest in developing countries.

The Fund’s investment advisor, Thornburg Investment Management, Inc. (“Thornburg”) intends to invest in companies that it believes will have growing revenues and earnings. The Fund can invest in companies of any size, from larger, well-established companies to smaller, emerging growth companies.

Thornburg primarily uses individual issuer and industry analysis to make investment decisions. Among the specific factors considered by Thornburg in identifying securities for inclusion in the Fund are:

•earnings growth potential

•business model

•industry growth potential

•industry leadership

•asset appreciation potential

•potential size of business

•price/earnings ratio

•price/revenue ratio

•PE/growth rate ratio

•price/cash flow ratio

•enterprise value/EBITDA (earnings before interest, taxes, depreciation and amortization) ratio

•management strength

•debt/capital ratio

The Fund typically makes equity investments in the following three types of companies:

Growth Industry Leaders: Companies in this category often have leadership positions in growing markets. In some cases these companies may have dominant market share. These companies tend to be larger and more established.

Consistent Growers: Companies in this category generally exhibit steady earnings or revenue growth, or both. These companies may have subscription or other recurring revenue profiles. Given their business models, these companies may outperform in weak markets.

Emerging Growth Companies: Companies often addressing a new market or carving out a niche in an existing market. Companies in this category may experience rapid growth, and tend to be smaller, earlier stage companies. These companies may exhibit high volatility.

Fund Summary Growth Fund

Inclusion of any investment in any of the three described categories represents the opinion of the advisor concerning the characteristics and prospects of the investment. There is no assurance that any company selected for investment will, once categorized in one of the three described investment categories, continue to have the positive characteristics or fulfill the expectations that the advisor had for the company when it was selected for investment, and any such company may not grow or may decline in earnings and size.

In conjunction with individual issuer analysis, Thornburg may identify and invest at times with a greater emphasis in industries or economic sectors it expects to experience growth. This approach may at times produce a greater emphasis on investment in certain industries or economic sectors, such as technology, financial services, healthcare or biotechnology. The Fund does not have a strategy to invest in particular industry or economic sectors, and its exposures to particular industries or economic sectors are expected to vary over time. Investment decisions are also based on domestic and international economic developments, outlooks for securities markets, interest rates and inflation, and the supply and demand for debt and equity securities.

Debt obligations, usually with associated equity features, occasionally will be considered for investment when Thornburg believes them to be more attractive than equity alternatives. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase.

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies, including smaller, earlier stage companies, may involve additional risks. These risks may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection. These risks may be more pronounced for investments in developing countries.

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those debt obligations may decline and the Fund’s share value and any dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated debt obligation (including particularly “junk” or “high yield” bonds) to pay principal and interest when due is typically less certain than for an issuer of a higher-rated debt obligation, lower-rated and unrated debt obligations are generally more vulnerable than higher-rated debt obligations to default, to ratings downgrades, and to liquidity risk.

Fund Summary Growth Fund

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term debt obligations. Decreases in market interest rates may result in prepayments of debt obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61.

Past Performance of the Fund

The following information provides some indication of the risks of investing in Growth Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares vary in each full year shown. The average annual total return figures compare Class R3, Class R4 and Class R5 share performance to the Russell 3000 Total Return Growth Index, a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Average Annual Total Returns

(periods ended 12-31-19)

Fund Summary Growth Fund

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Manager:

Ted Chang, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2019.

Greg Dunn, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2012.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

International Growth Fund

Investment Goal

The Fund seeks long-term growth of capital by investing in equity securities selected for their growth potential.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, Class R5 and Class R6 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.50%, 1.40%, 0.99%, and 0.89%, respectively. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

Fund Summary International Growth Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 25.83% of the average value of its portfolio.

Principal Investment Strategies

The Fund expects to invest primarily in equity securities from issuers around the world (primarily common stocks) selected for their growth potential and, under normal market conditions, invests at least 75% of its assets in foreign securities or depository receipts of foreign securities. However, the Fund may own a variety of securities, including domestic equity securities. The Fund may invest in developing countries.

The Fund’s investment advisor, Thornburg Investment Management, Inc. (“Thornburg”) intends to invest in companies that it believes will have growing revenues and earnings. The Fund can invest in companies of any size, from larger, well-established companies to smaller, emerging growth companies.

Thornburg primarily uses individual issuer and industry analysis to make investment decisions. Among the specific factors considered by Thornburg in identifying securities for inclusion in the Fund are:

•earnings growth potential

•business model

•industry growth potential

•industry leadership

•asset appreciation potential

•potential size of business

•price/earnings ratio

•price/revenue ratio

•PE/growth rate ratio

•price/cash flow ratio

•enterprise value/EBITDA (earnings before interest, taxes, depreciation and amortization) ratio

•management strength

•debt/capital ratio

The Fund typically makes equity investments in the following three types of companies:

Growth Industry Leaders: Companies in this category often have leadership positions in growing markets. In some cases these companies may have dominant market share. These companies tend to be larger and more established.

Fund Summary International Growth Fund

Consistent Growers: Companies in this category generally exhibit steady earnings or revenue growth, or both. These companies may have subscription or other recurring revenue profiles. Given their business models, these companies may outperform in weak markets.

Emerging Growth Companies: Companies often addressing a new market or carving out a niche in an existing market. Companies in this category may experience rapid growth, and tend to be smaller, earlier stage companies. These companies may exhibit high volatility.

Inclusion of any investment in any of the three described categories represents the opinion of the advisor concerning the characteristics and prospects of the investment. There is no assurance that any company selected for investment will, once categorized in one of the three described investment categories, continue to have the positive characteristics or fulfill the expectations that the advisor had for the company when it was selected for investment, and any such company may not grow or may decline in earnings and size.

In conjunction with individual issuer analysis, Thornburg may identify and invest at times with a greater emphasis in industries or economic sectors it expects to experience growth. This approach may at times produce a greater emphasis on investment in certain industries or economic sectors, such as technology, financial services, healthcare or biotechnology. The Fund does not have a strategy to invest in particular industry or economic sectors, and its exposures to particular industries or economic sectors are expected to vary over time. Investment decisions are also based on domestic and international economic developments, outlooks for securities markets, interest rates and inflation, and the supply and demand for debt and equity securities.

Debt obligations, usually with associated equity features, occasionally will be considered for investment when Thornburg believes them to be more attractive than equity alternatives. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase.

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection.

Developing Country Risk – The risks which may affect investments in foreign issuers (see “Foreign Investment Risk,” above) may be more pronounced for investments in developing countries because the economies of those countries are usually less diversified, communications, transportation and economic infrastructures are less developed, and developing countries ordinarily have less established legal, political, business and social frameworks. At times the prices of equity securities or debt obligations of a developing country issuer may be extremely volatile. An issuer domiciled in a developed country may be similarly affected by these developing country risks to the extent that the issuer conducts its business in developing countries.

Fund Summary International Growth Fund

Risks Affecting Specific Countries or Regions – If a significant portion of the Fund’s assets is invested in issuers that are economically exposed to one country or region, the Fund’s share value may be more susceptible to the conditions and developments in that country or region, and potentially more volatile than the share value of a more geographically diversified fund. The nature and degree of the risks affecting a given country or region, and the extent of the Fund’s exposure to any such country or region, is expected to vary over time.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies, including smaller, earlier stage companies, may involve additional risks. These risks may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those debt obligations may decline and the Fund’s share value and any dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated debt obligation (including particularly “junk” or “high yield” bonds) to pay principal and interest when due is typically less certain than for an issuer of a higher-rated debt obligation, lower-rated and unrated debt obligations are generally more vulnerable than higher-rated debt obligations to default, to ratings downgrades, and to liquidity risk.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term debt obligations. Decreases in market interest rates may result in prepayments of debt obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices. This risk may be more pronounced for the Fund’s investments in developing countries.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 61.

Past Performance of the Fund

The following information provides some indication of the risks of investing in International Growth Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares vary in each full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to the MSCI ACWI ex USA Growth Net Total Return USD Index, a market capitalization weighted index which includes growth companies in developed and emerging markets throughout the world, excluding the United States. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Fund Summary International Growth Fund

Average Annual Total Returns

(periods ended 12-31-19)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Fund Summary International Growth Fund

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Greg Dunn, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2012.

Sean Sun, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2017.

Purchase and Sale of Fund Shares

Employer-sponsored retirement plans wishing to make shares of the Fund available to plan participants should contact a financial intermediary authorized to sell shares of the Funds. As a participant in an employer-sponsored retirement plan that makes Fund shares available, you may add shares to your account by contacting your plan administrator. Although the Funds do not currently impose any investment minimums on the purchase of shares through an employer-sponsored retirement plan, the plan itself may establish such minimums. Contact your plan administrator for more information.

Please contact your retirement plan administrator if you wish to sell your shares. Your plan administrator will conduct the transaction for you, or provide you with the means to conduct the transaction yourself.

Tax Information

Fund distributions to qualified retirement plan accounts, and transactions in Fund shares by those accounts, are not generally subject to current federal income tax under existing federal law. Please see “Taxes” on page 82 of this Prospectus for additional information. Purchasers are cautioned to seek the advice of their own advisors about the tax consequences of contributions to plan accounts and distributions from plan accounts.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase shares of a Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund, its investment advisor and/or its distributor may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary’s website for more information.

Fund Summary

Income Builder Fund

Investment Goal

The Fund’s primary investment goal is to provide a level of current income which exceeds the average yield on U.S. stocks generally, and which will generally grow, subject to periodic fluctuations, over the years on a per share basis. The Fund’s secondary investment goal is long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees

(fees paid directly from your investment)

Annual Fund Operating Expenses

(expenses that you pay each year as a percentage of the value of your investment)

(1)Thornburg Investment Management, Inc. (“Thornburg”) has contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class R3, Class R4, Class R5, and Class R6 expenses (excluding taxes, interest expenses, 12b-1 distribution and service fees, acquired fund fees and expenses, brokerage commissions, borrowing costs, expenses relating to short sales, and extraordinary expenses such as litigation costs) do not exceed 1.50% 1.40%, 0.99%, and 0.80% respectively, not including the effects of Acquired Fund Fees and Expenses. The agreement to waive fees and reimburse expenses may be terminated by the Fund’s Trustees at any time, but may not be terminated by Thornburg before February 1, 2021, unless Thornburg ceases to be the investment advisor of the Fund prior to that date. Thornburg may recoup amounts waived or reimbursed during the Fund’s fiscal year if actual expenses fall below the expense cap during that same fiscal year.

(2)The figures for Total Fund Operating Expenses After Fee Waiver/Expense Reimbursement in this table have been recalculated to add amounts for “Acquired Fund Fees and Expenses,” in accordance with regulatory rules. Acquired Fund Fees and Expenses vary with changes in the amount of the Fund’s investments in investment companies and other factors. Please see the disclosure under the caption “Explanation of Acquired Fund Fees and Expenses” for a further explanation.

Fund Summary Income Builder Fund

Example. This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.

The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:

Explanation of Acquired Fund Fees and Expenses. “Acquired Fund Fees and Expenses” shown in the Annual Fund Operating Expenses table are expenses incurred indirectly by other investment companies, such as business development companies, in which the Fund may hold shares. These operating expenses are similar to the expenses paid by other businesses owned by the Fund, are not direct costs paid by Fund shareholders, and are not used to calculate the Fund’s net asset value. These expenses have no impact on the costs associated with Fund operations. Regulatory rules require that the Acquired Fund Fees and Expenses be added to the actual operating expenses of the Fund, and that the total be shown in the bottom line of the Annual Fund Operating Expenses table. Please see the expense figures shown in the Financial Highlights for the Fund, at pages 98–99, for a clearer picture of the Fund’s actual operating costs.

Portfolio Turnover. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over”) its portfolio. A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 43.69% of the average value of its portfolio.

Principal Investment Strategies

The Fund pursues its investment goals by investing in a broad range of income producing securities, primarily including stocks and bonds, as described below. The Fund will under normal conditions invest at least 80% of its assets in income-producing securities, and at least 50% of its assets in common stocks.

The Fund may invest in any stock or other equity security which the investment advisor believes may assist the Fund in pursuing its investment goals (including smaller companies with market capitalization of less than $500 million and companies in developing countries), including preferred stock and publicly traded real estate investment trusts. The Fund expects that equity investments in the Fund’s portfolio normally will be weighted in favor of companies which pay dividends or other current income.

The Fund may invest in debt obligations of any kind, including corporate bonds and other obligations, mortgage- and other asset-backed securities and government obligations. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase. The Fund also may invest in debt obligations which have a combination of equity and debt characteristics, such as convertible bonds.

The Fund may invest a significant portion of its assets in securities of issuers domiciled in or economically tied to countries outside the United States, including developing countries.

The Fund’s investments are determined by individual issuer and industry analysis. Investment decisions are based on domestic and international economic developments, outlooks for securities markets, interest rates and inflation, the supply and demand for debt and equity securities, and analysis of specific issuers. The Fund ordinarily acquires and holds debt obligations for investment rather than for realization of gains by short-term trading on market fluctuations. However, the Fund

Fund Summary Income Builder Fund

may dispose of any such security prior to its scheduled maturity to enhance income or reduce loss, to change the portfolio’s average maturity, or otherwise to respond to market conditions.

Principal Investment Risks

An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund.

Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests.

Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies may involve additional risks, which may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those obligations may decline and the Fund’s share value and the dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated obligation to pay principal and interest when due is typically less certain than for an issuer of a higher-rated obligation, lower-rated and unrated obligations are generally more vulnerable than higher-rated obligations to default, to ratings downgrades, and to liquidity risk.

High Yield Risk – Debt obligations that are rated below investment grade and unrated obligations of similar credit quality (commonly referred to as “junk” or “high yield” bonds) may have a substantial risk of loss. These obligations are generally considered to be speculative with respect to the issuer’s ability to pay interest and principal when due. These obligations may be subject to greater price volatility than investment grade obligations, and their prices may decline significantly in periods of general economic difficulty or in response to adverse publicity, changes in investor perceptions or other factors. These obligations may also be subject to greater liquidity risk.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term obligations. This effect is also typically more pronounced for mortgage- and other asset-backed securities, the value of which may fluctuate more significantly in response to interest rate changes. When interest rates decrease, the Fund’s dividends may decline.

Prepayment and Extension Risk – When market interest rates decline, certain debt obligations held by the Fund may be repaid more quickly than anticipated, requiring the Fund to reinvest the proceeds of those repayments in obligations which bear a lower interest rate. Conversely, when market interest rates increase, certain debt obligations held by the Fund may be repaid more slowly than anticipated, causing assets of the Fund to remain invested in relatively lower yielding obligations. These risks may be more pronounced for the Fund’s investments in mortgage-backed and asset-backed securities.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection. In addition, some foreign government debt obligations may be subject to default, delays in payment, adverse legislation or government action, or could be downgraded by ratings agencies.

Fund Summary Income Builder Fund

Developing Country Risk – The risks which may affect investments in foreign issuers (see “Foreign Investment Risk,” above) may be more pronounced for investments in developing countries because the economies of those countries are usually less diversified, communications, transportation and economic infrastructures are less developed, and developing countries ordinarily have less established legal, political, business and social frameworks. At times the prices of equity securities or debt obligations of a developing country issuer may be extremely volatile. An issuer domiciled in a developed country may be similarly affected by these developing country risks to the extent that the issuer conducts its business in developing countries.

Risks Affecting Specific Countries or Regions – If a significant portion of the Fund’s assets is invested in issuers that are economically exposed to one country or region, the Fund’s share value may be more susceptible to the conditions and developments in that country or region, and potentially more volatile than the share value of a more geographically diversified fund. The nature and degree of the risks affecting a given country or region, and the extent of the Fund’s exposure to any such country or region, is expected to vary over time.

Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of its investments promptly, or may only be able to sell investments at less than desired prices.

Real Estate Risk – The Fund’s investments in publicly traded real estate investment trusts (“REITs”) are subject to risks affecting real estate investments generally (including market conditions, competition, property obsolescence, changes in interest rates and casualty to real estate), as well as risks specifically affecting REITs (the quality and skill of REIT management and the internal expenses of the REIT).

Additional information about Fund investments, investment strategies and risks of investing in the Fund appears below beginning on page 61.

Past Performance of the Fund

The following information provides some indication of the risks of investing in Income Builder Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class R3 shares vary in each full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to a Blended Benchmark, comprised of 25% Bloomberg Barclays US Aggregate Bond Total Return Value USD, which represents a broad measure of bond market performance, and 75% MSCI World Net Total Return USD Index, which represents a broad measure of equity market performance in developed markets. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2019. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.

Annual Total Returns – Class R3 Shares

Fund Summary Income Builder Fund

Average Annual Total Returns

(periods ended 12-31-19)

After-tax returns are calculated using the highest historical individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns shown relate only to Class R3 shares, and after-tax returns for other share classes will vary.

Management

Investment Advisor: Thornburg Investment Management, Inc.

Portfolio Managers:

Jason Brady, cfa, the president of the Trust and the chief executive officer, president, and a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2007.

Matt Burdett, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2019.

Ben Kirby, cfa, a managing director and head of investments of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since 2013.

Brian J. McMahon, the vice chairman of the Trust and a managing director and chief investment strategist of Thornburg, has been one of the persons jointly and primarily responsible for management of the Fund since its inception.