5 CFR § 1655.7 - Interest rate.
(a) Except as provided in paragraph (b) of this section, loans will bear interest at the monthly G Fund interest rate established by the Department of the Treasury in effect on the 15th of the month prior to the date the loan request is made.
(b) The interest rate calculated under this section remains fixed until the loan is repaid, unless a civilian participant informs the TSP record keeper that he or she entered into active duty military service, and, as a result, requests that the interest rate on a loan issued before entry into active duty military service be reduced to an annual rate of 6 percent for the period of such service. The civilian participant must provide the record keeper with the beginning and ending dates of active duty military service.