A 67-Year-Old Cashed Out Her 401(k) to Wipe Out the Mortgage, Then Medicare Added $487 a Month Two Years Later

The moment she signed the check that cleared her mortgage, a 67-year-old retiree felt the relief only a debt-free homeowner knows. She had pulled a single large lump sum from her traditional 401(k), wired it to the lender, and finally owned the house outright. Two years later, her Social Security deposit shrank without warning. Medicare had added $487 a month to her Part B premium, traced straight back to that satisfying payoff.

This scenario is more common than many retirees realize. Versions of this story appear regularly on retirement forums: someone clears the mortgage with one big withdrawal, then opens a letter from Social Security a couple of years later and wonders what happened. The culprit has a clunky name, the Income-Related Monthly Adjustment Amount, or IRMAA, and it catches careful, responsible savers more than almost anyone else.

Why a Debt-Free Move Triggered a Medicare Bill

Medicare looks backward. The 2026 Part B premium is based on income reported on the 2024 tax return, and that two-year lookback is where the trap lives. A traditional 401(k) withdrawal counts as fully taxable ordinary income, so a lump sum large enough to retire a mortgage often lands in a single tax year and pushes Modified Adjusted Gross Income (MAGI) into territory the retiree would otherwise never approach. One detail surprises many people: MAGI for IRMAA purposes includes not just wages, pensions, and retirement distributions, but also tax-exempt interest from municipal bonds. A retiree who holds a substantial muni portfolio for its tax advantages may still find that interest contributing to a surcharge tier.

IRMAA is a cliff system, and a steep one. One dollar over a threshold triggers the full tier’s surcharge, not a prorated fraction of it. The standard 2026 Part B premium is $202.90 a month, up from $185.00 in 2025. Surcharges begin once MAGI crosses $109,000 for a single filer or $218,000 for a joint return. The top tier, which starts at $500,000 for a single filer or $750,000 for a joint return, adds $487 per month on top of that base. That top bracket is frozen through at least 2028, meaning it will not be indexed for inflation until Congress acts. Stack a six-figure 401(k) withdrawal on existing Social Security, pension, and investment income, and a retiree who normally lives modestly can land in the highest bracket for one unusual year. IRMAA also applies to Part D drug coverage, so the total monthly surcharge bite at the top tier can run well over $500 when both Part B and Part D are counted. About 5.1 million Medicare beneficiaries paid Part B IRMAA surcharges in 2025, roughly 7% of all enrollees. That number illustrates just how many otherwise moderate-income retirees a single large distribution can sweep into surcharge territory.

Because Medicare premiums are deducted directly from the Social Security check, the hit arrives as a smaller deposit rather than a separate bill. That quiet subtraction is what makes the surprise sting.

The One Detail That Drives the Whole Outcome

The single most important variable is not the size of the mortgage or the interest rate. It is whether the payoff happens in one tax year or two.

Picture a $300,000 mortgage balance. Pulled from a traditional 401(k) in one calendar year, the full amount lands on a single tax return and rockets MAGI past thresholds the retiree would otherwise never approach. Split across late December and early January, the same payoff hits two separate returns, and each year’s MAGI may stay safely below the next IRMAA cliff. Same house, same lender, same total dollars paid. Wildly different Medicare bill.

Roth withdrawals offer another lever. Qualified Roth distributions do not count toward MAGI, so drawing from a Roth account adds nothing to the IRMAA calculation. Drawing principal from a regular taxable brokerage account also avoids adding to MAGI beyond any realized gains. A blended payoff, combining some Roth, some taxable savings, and some traditional funds spread over two years, can often clear the mortgage without brushing a surcharge tier.

One development worth flagging for 2025 filers: the One Big Beautiful Bill Act, signed into law on July 4, 2025, created a new $6,000 bonus deduction for taxpayers age 65 and older. The deduction phases out for single filers with MAGI above $75,000 (fully eliminated at $175,000) and for joint filers above $150,000 (fully eliminated at $250,000). Critically, the deduction sits below the AGI line, reducing taxable income but not MAGI itself. Because IRMAA is calculated against MAGI, a retiree hoping that new write-off will pull them under a surcharge threshold will be disappointed. The calculation Medicare uses is unaffected, regardless of whether the retiree qualifies for the full deduction, a reduced amount, or nothing at all.

How This Lands Alongside the Rest of Retirement

Once a surcharge is in place, the calendar becomes the primary relief valve. IRMAA is recalculated every year, so when income returns to its normal level, the following year’s premium drops back to the standard amount. The financial hit is real, but it is typically temporary, often confined to a single year tied to that one outsized withdrawal.

Appealing is the part people consistently get wrong. The Social Security Administration’s SSA-44 form covers eight specific life-changing events: marriage, divorce or annulment, death of a spouse, work stoppage, work reduction, loss of income-producing property, loss of pension income, and an employer settlement payment related to employer bankruptcy or reorganization. Choosing to pay off a mortgage is not on that list. The surcharge on a voluntary lump-sum withdrawal has no qualifying appeal path. The fix has to happen before the withdrawal, not after.

What to Think Through Before Pulling the Trigger

Every lever here is available before the withdrawal, and none of them work once the check has cleared.

1. Before any large traditional 401(k) withdrawal, run the MAGI math against the IRMAA thresholds for both Part B and Part D. A payoff that saves a few thousand in mortgage interest but triggers a five-figure tax bill and a Medicare surcharge is not the win it looks like on paper.
2. If the goal is to be debt-free, spread the withdrawal across two tax years, or blend in Roth and taxable-account dollars. The mortgage still goes away. The surcharge does not have to come with it.

Every retiree’s tax picture is a little different, and the line between a clean payoff and an expensive one can come down to which December week the check is signed. A short conversation with a tax preparer before the withdrawal almost always pays for itself.

Editor’s note: This pass added that IRMAA surcharges apply to Part D drug coverage in addition to Part B, noted the phase-out range for the One Big Beautiful Bill Act senior deduction ($75,000 to $175,000 for single filers, $150,000 to $250,000 for joint filers), and clarified that the deduction reduces taxable income but not MAGI, leaving IRMAA calculations unaffected even for seniors who qualify for the full amount.

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