Employers must withhold this tax from wages or compensation when they
pay employees more than $200,000 in a calendar year. The withholding is not segregated from your other tax withholdings by your employer or by the IRS.
Within TurboTax, when you prepare Form 8959, the amount withheld for additional Medicare tax by your employer is transferred to the Tax Payments Worksheet, for inclusion in line 18 of Form 1040.
When you complete your 2018 Form 1040, with Form 8959 attached, you are paying the additional Medicare tax, but receiving credit for the withholding that was already done through your employer's payroll.
