5 CFR §1655.17

1. (a)A participant may repay a loan in full, without a penalty, at any time before the declaration of a deemed distribution or loan foreclosure under § 1655.15. Repayment in full means receipt by the TSP record keeper of a payment, by check or guaranteed funds made payable to the Thrift Savings Plan or via loan direct debit repayments, of all principal and interest due on the loan.
2. (b)If a participant returns a loan check to the TSP record keeper, it will be treated as a repayment; however, additional interest may be owed, which, if not paid, could result in a deemed distribution. The loan, even though repaid, will also be taken into account in determining the maximum amount available for future loans, in accordance with § 1655.6(b).
3. (c)The amount outstanding on a loan can be obtained from the TSP website, the ThriftLine, or by a written request to the TSP record keeper.