There are additional Medicare taxes for high earners. Most employers do not withhold the full amount if you fall into the high earner category. Please see IRS website https://www.irs.gov/businesses/small-businesses-self-employed/questions-and-answers-for-the-addition....
You do not need to be self-employed to owe this extra tax - it also applies to wages.
Quote from IRS website "Additional Medicare Tax went into effect in 2013 and applies to wages, compensation, and self-employment income above a threshold amount received in taxable years beginning after December 31, 2012."
