How do I report 401k withdrawals to Medicare if the IRS does not report it?
You do not need to report your income to Medicare. Medicare looks at your tax returns to calculate the income-based portion of your premiums.
As a result, if you take taxable distributions from your 401(k), then you will report this on your tax return, and if your total income is high enough, your Medicare premiums will adjust.
Non-taxable distributions (such as a Roth IRA or Roth 401(k)) do not affect your Medicare premiums.