From tsp.gov:
https://www.tsp.gov/publications/tsplf15.pdf
You Have Options.
Now that you’ve left federal service, you may be
considering what to do about your TSP account. This
leaflet explains the options you can choose from. Be
aware that if you’re under age 55 and make a withdrawal,
you may be subject to the IRS 10% early withdrawal
penalty tax. For more information and exceptions to
the rule, please read the TSP tax notice Important Tax
Information About Payments From Your TSP Account.
https://www.tsp.gov/publications/tsp-536.pdf
b. Additional 10% Early Withdrawal Penalty Tax
If you receive a TSP distribution before you reach age
59½, in addition to the regular income tax, you may have
to pay an early withdrawal penalty tax equal to 10% of
any taxable portion of the distribution not transferred or
rolled over. The additional 10% tax generally does not
apply to
• payments made after you separate from service during
or after the year you reach age 55 (or the year you reach
age 50 if you are a public safety employee as defined in
section 72(t)(10)(B)(ii) of the Internal Revenue Code);
• up to $5,000 of any payment received within one year
following a birth or qualified adoption in accordance
with section 72(t)(2)(H) of the Internal Revenue Code;
• annuity payments;
• automatic enrollment refunds;
• payments resulting from total and permanent
disability;3
• payments resulting from death;
• payments made from a beneficiary participant account;
• payments made in a year you have deductible medical
expenses that exceed 7.5% of your adjusted gross income;
• payments ordered by a domestic relations court; or
• substantially equal payments over your life expectancy.
