From the TSP Booklet on withdrawing funds after separation, available on TSP website:
Page 4:
Full Withdrawal
When you are ready to choose among the withdrawal options for your entire account balance, there are a number
of ways you can do so .
A single payment. You can withdraw your entire TSP
account balance in a single payment .
A series of monthly payments. You can withdraw your
TSP account in a series of substantially equal monthly
payments until it is completely paid out . You can choose:
• Monthly payments computed by the TSP based on
IRS life expectancy tables. Your initial payment
amount will be based on your age and your account
balance at the time of the first payment . Each year,
on the anniversary of the date of your first monthly
payment, the TSP will recalculate the amount of
your monthly payments . The recalculation will be
based on your age and your account balance at the
end of the preceding year .
• A specific dollar amount. You will receive payments
in the amount that you request until your entire account balance has been paid to you . The amount of
each monthly payment must be $25 or more .
You can use the monthly payment calculators on the TSP
website to estimate how many monthly payments you
can receive from your account when you choose a specific
dollar amount, or to estimate how much you can receive
each month if you choose monthly payments based on
your life expectancy . Remember that investment gains or
losses could cause your account balance to increase or decrease, which could expand or reduce either the amount
of your monthly payments or their duration
and on page 6:
However, if you are receiving a series of monthly payments, you can at any time change to a final single payment, or change where or how your payments are sent .
Refer to the TSP website for detailed instructions . In
addition, during the annual change period at the end
of each calendar year, you can change the dollar amount
of your payments, and you can make a one-time-only
change from TSP-computed payments to a specified
dollar amount . When you make a change during the
annual change period, the TSP must receive Form
TSP-73, Change in Monthly Payment Amount, from you
by December 15 for the change to be effective with the
first payment you receive after December 31