Per IRS:
Employers are responsible for withholding the 0.9% Additional Medicare tax on an individual's wages paid in excess of $200,000 in a calendar year, without regard to filing status.
If you overpaid your Additional Medicare Tax, you may be able to receive a tax refund or have the overpayment applied against any taxes you may owe. TurboTax will automatically add form 8959 when needed.
Once you complete Form 8959 and figure out the total Additional Medicare Tax you're responsible for, the final section of the form subtracts the tax you paid through paycheck withholding as shown on your W-2. The result shows if there is any Additional Medicare Tax due—which ultimately gets reported on your 1040 form line 25c.
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