Summary Prospectus by Investment Company - Form 497K

For the Fund's Prospectus or SAI, visit www.thornburg.com/download

Thornburg International Growth FundSummary Prospectus​

Before you invest, you may want to review the Fund's Prospectus and Statement of Additional Information (SAI), which contain more information about the Fund and its risks. You can find the Fund's Prospectus, SAI and other information about the Fund online at www.thornburg.com/download. You can also get this information at no cost by calling 800.847.0200 or by sending an e-mail request to [email protected]. The current Prospectus and SAI, dated February 1, 2024, as supplemented, are incorporated by reference into this Summary Prospectus.

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. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.

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 unusual expenses such as contingency fees or 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, 2025, 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.

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 37.24% of the average value of its portfolio.

Thornburg International Growth FundSUMMARY PROSPECTUSFebruary 1, 2024, AS SUPPLEMENTED

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For the Fund's Prospectus or SAI, visit www.thornburg.com/download

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 net assets in foreign securities or depositary receipts of foreign securities. However, the Fund may own a variety of securities, including domestic equity securities. The Fund may invest in developing countries. As used herein, the term "net assets" includes the amount of any borrowings made by the Fund for investment purposes.

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.

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 securities, and analysis of specific issuers.

Thornburg typically categorizes the Fund's equity investments in one of the following three categories:

Growth Industry Leader: 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 Grower: 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 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 Thornburg's opinion 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 Thornburg 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 securities.

The Fund may sell an investment if Thornburg has identified a better investment opportunity, in response to changes in the conditions or business of the investment's issuer or changes in overall market conditions, if Thornburg has a target price for the investment and that target price has been achieved, or if, in Thornburg's opinion, the investment no longer serves to achieve the Fund's investment goals.

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. The risks are presented in alphabetical order to facilitate readability, and their order does not imply that the realization of one risk is more likely to occur or have a greater adverse impact than another risk. The relative significance of each risk below may change over time.

Credit Risk - The inability of an issuer to pay principal and interest on its debt obligations when due, or the downgrading of an issuer's debt obligations by ratings agencies, may adversely affect the market's perceptions of the issuer's financial strength and may therefore result in declines in the issuer's stock price.

Cybersecurity and Operational Risk - Operational failures, cyber-attacks or other disruptions that affect the Fund's service providers, the Fund's counterparties, other market participants or the issuers of securities held by the Fund may adversely affect the Fund and its shareholders, including by causing losses for the Fund or impairing Fund operations.

Developing Country Risk - The risks which may affect investments in foreign issuers (see "Foreign Investment Risk," below) 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 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.

Equity Risk - The value of the Fund's equity investments may fluctuate significantly over time in response to factors affecting individual issuers, particular industries, or the market as a whole. Additionally, common stock ranks below preferred stock and debt securities in claims for dividends and for assets of a company in a liquidation or bankruptcy.

Foreign Currency Risk - Fluctuations in currency exchange rates can adversely affect the value of the Fund's foreign investments. Such fluctuations may occur for a number of reasons, including market and economic conditions, or a government's decision to devalue its currency or imposecurrency controls.

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.

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.

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SUMMARY PROSPECTUSFebruary 1, 2024, AS SUPPLEMENTEDThornburg International Growth Fund

For the Fund's Prospectus or SAI, visit www.thornburg.com/download

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.

Redemption Risk - If a significant percentage of the Fund's shares is owned or controlled by a single shareholder, the Fund is subject to the risk that a redemption by that shareholder of all or a large portion of its shares may require the Fund to sell securities at less than desired prices, and the Fund's remaining shareholders may also incur additional transaction costs or adverse tax consequences from such trading activity.

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. A specific country or region could also be adversely affected by conditions or developments arising in other countries. For example, the U.S. government could take actions to prohibit or restrict individuals or companies within the U.S. from purchasing or holding the shares of issuers in another country, which may limit the Fund's ability to invest in that country or cause the Fund to have to sell investments in that country at less than desired prices. 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.

Risks Affecting Specific Issuers - The value of a security may decline in response to developments affecting the specific issuer of the 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.

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.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears beginning on page 66 of the Prospectus.

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 All Country World ex USA Net Total Return, a broad-based securities market index that represents the overall applicable market in which the Fund invests, and the MSCI All Country World ex USA Growth Net Total Return USD Index, an additional index that represents the market sectors which Thornburg believes are more representative of the Fund's investment universe. These indexes are not actively managed and are not available for direct investment. The bar charts and performance tables assume reinvestment of dividends and distributions. 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, 2023. Updated performance information may be obtained on the Thornburg website at Thornburg.com or by calling 1-800-847-0200.

Annual Total Returns - Class R3 Shares

Thornburg International Growth FundSUMMARY PROSPECTUSFebruary 1, 2024, AS SUPPLEMENTED

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For the Fund's Prospectus or SAI, visit www.thornburg.com/download

Average Annual Total Returns

(periods ended 12-31-23)

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:

Nicholas Anderson, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for the day-to-day management of the Fund since 2021.

Emily Leveille, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for the day-to-day management of the Fund since 2021.

Sean Sun, cfa, a managing director of Thornburg, has been one of the persons jointly and primarily responsible for the day-to-day 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 89 of the 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.

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SUMMARY PROSPECTUSFebruary 1, 2024, AS SUPPLEMENTEDThornburg International Growth Fund

For the Fund's Prospectus or SAI, visit www.thornburg.com/download

9/30/24
TH2257

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.