It was probably an offset. If you owed money to the IRS they take what you owe and give you the balance.
You should be getting a letter from the IRS explaining the difference in your refund amount. It can take a couple weeks to receive.
Please see the TurboTax FAQ below for information on why your refund was lower than expected:
Why is my direct-deposited refund or check lower than the amount in TurboTax?