IRA conversion to Roth and affect on Medicare Premium

The taxable amount of a Roth conversion will correspondingly  increase your AGI used to determine the IRMAA adjustment to your cost of Medicare Part B and D premiums for the year two years after the year of your tax return that includes the conversion.  The change in your Medicare premiums will depend on the amount of change in your AGI.  You can find the IRMAA AGI thresholds on the Medicare.gov website:

Part B:  https://www.medicare.gov/your-medicare-costs/part-b-costs

Part D:  https://www.medicare.gov/drug-coverage-part-d/costs-for-medicare-drug-coverage/monthly-premium-for-drug-plans

and you thought a Roth IRA was tax free .

But after a while your medicare premium will go back down.  unless you keep on doing conversions.