By now many readers have filed their federal income tax returns, and some may even have received (and spent) their refunds. Refunds are a good thing, and spending them is even better; what hurts is finding out that you owe more money than you thought you owed, and ending up having to make a large tax payment with, perhaps, a penalty thrown in.

Believe it or not, your TSP withdrawals can be a cause of your balance due on your federal income tax. If you are taking substantially equal monthly payments, with few exceptions, federal income taxes are being withheld as if you were married, filing jointly and claiming three dependents.

This rate of withholding is not enough, by a long shot, to cover the taxes actually due on your TSP withdrawals. If you do not have 90% of the taxes you owe withheld by the end of the tax year in question, you are subject to the 10% estimated tax penalty (10% of the difference between what you had withheld and the 90% amount).

If you find yourself in a situation where you owe more this year than you expected, try increasing your withholding on your TSP withdrawals. That’s as easy as preparing a form W-4P, Withholding Certificate for Pension or Annuity Payments, and submitting it to the TSP. The form can be found on the website of the Internal Revenue Service, or in the “forms and publications” section of the TSP website. Expect a revision of withholding forms, if not a revision in the default withholding amount as a result of the recently passed tax reform legislation.

If you want to stay on top of the tax rules for the TSP, read the TSP’s informative publication Tax Information: Payments from Your TSP Account, which is available in the “forms and publications section of the TSP website.

Here’s another tax tidbit: If you were deployed and made any contributions to your traditional TSP from tax-exempt income, the portion of your future withdrawal(s) that is based on those contributions is not taxed when withdrawn.