Will a large Roth conversion push me into higher Medicare premiums two years later?

Yes, and it catches many retirees off guard. A Roth conversion adds to your modified adjusted gross income for the year, and Medicare's IRMAA surcharges on Part B and Part D premiums are based on your MAGI from two years prior. A big one-time conversion can spike your income past an IRMAA threshold, raising your premiums for a full year once the surcharge kicks in. The Social Security Administration administers these income-related adjustments. To avoid a nasty surprise, conversions are often best done before you enroll in Medicare at 65, or sized carefully to stay under the relevant bracket. Weigh the conversion against these hidden costs, not just your income-tax bracket.