RHYTHM PHARMACEUTICALS, INC.

(a)Disclosure in lieu of reporting on a Current Report on Form 8-K.

The Information set forth below is included for the purpose of providing disclosure under Item 1.01 – “Entry into a Material Definitive Agreement” and Item 2.03 – Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant” of Form 8-K.

On November 2, 2026, we entered into the LGC Amendment, the Loan Agreement and the A&R RIFA. For more information on the LGC Amendment, Loan Agreement and A&R RIFA for purposes of Item 1.01 of Form 8-K and the Loan Agreement and the A&R RIFA for purposes of Item 2.03 of Form 8-K, see Note 16. Subsequent Events, to our unaudited condensed consolidated financial statements. .

(b)Material changes to the procedures by which security holders may recommend nominees to the board of directors.

None.

(c)Insider Trading Arrangements and Policies.

On August 6, 2026, David McGirr, the Company's Director, adopted a trading plan intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act for the sale of up to 21,810 shares of the Company's common stock until February 7, 2027.

On August 18, 2026, Christophe Jean, the Company's Director, adopted a trading plan intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act for the sale of up to 10,000 shares of the Company's common stock until July 17, 2027.

On September 9, 2026, Alastair Garfield, the Company's Chief Scientific Officer, adopted a trading plan intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act for the sale of up to 100% of the shares of our common stock issued to Dr. Garfield upon the settlement of outstanding restricted stock units, less the number of shares surrendered to the issuer or sold to cover tax withholding obligations in connection with the vesting and settlement of such restricted stock units, until February 1, 2028.

On September 18, 2026 , David Meeker, the Company's Chief Executive Officer, terminated a trading plan intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act, which was previously adopted on June 15, 2026 and provided for the sale of up to 300,000 shares of the Company's common stock until March 15, 2027.

Other than as disclosed above, during the three months ended September 30, 2026, no director or officer of the Company adopted orterminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408(a) of Regulation S-K.