From www.tsp.gov:
What are "catch-up contributions"?
Catch-up contributions are supplemental tax-deferred contributions available to TSP participants age 50 or older who are already contributing the
maximum amount of regular TSP contributions for which they are eligible, up to the maximum IRS elective deferral limit of $16,500 for 2009 and $16,500 for 2010. Catch-up contributions have their own annual limit of $5,500 for 2009 and $5,500 for 2010. Increases are indexed to inflation.
If you are eligible, you can submit an election at any time to make these contributions. Use Form
TSP-1-C, Catch-up Contribution Election, available in the Forms and Publications section of this Web site or from your agency. Some agencies may use an electronic version of the form. Check with your agency for guidance.
Your contributions are invested in the TSP funds according to your most recent contribution allocation. You can change, stop, or restart your catch-up contributions at any time. Your contributions will automatically stop at the end of the calendar year or when you reach the maximum dollar limit for the year. You must make a new election each calendar year. Agencies do not match catch-up contributions.
Your catch-up contributions are deducted from your basic pay each pay period; therefore, you must be currently employed by the Federal Government and receiving pay. In addition, you may not be in the 6-month non-contribution period following receipt of a TSP financial hardship in-service withdrawal. For more information, read the TSP Fact Sheet “
Catch-up Contributions.”