His Inflation-Indexed Bonds Gained $10,000 in Principal — and Medicare May Raise His Premiums Before He Sells

Quick Read

• TIPS held in taxable accounts can create phantom income: a $10,000 inflation adjustment is taxable even if the bond is not sold.

• A $10,000 TIPS adjustment could lift a single filer's MAGI from $105,000 to $115,000, which may trigger a $1,148 annual Medicare surcharge in 2028.

• Keeping TIPS in a traditional IRA removes the yearly taxable adjustment, postponing any Medicare premium effect until money is withdrawn.

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Picture a 66-year-old retiree who owns Treasury Inflation-Protected Securities, or TIPS, in a taxable brokerage account. He has not sold them, yet inflation has increased the bonds' principal by $10,000. The Internal Revenue Service generally treats that increase as income for the year, even though the money remains invested. The bonds still pay interest every six months, but the higher principal could also affect his Medicare premiums later.

Whether that becomes a problem depends on where his income already stands. The Centers for Medicare & Medicaid Services says income-related surcharges affect about 8% of Part B enrollees. In 2026, the surcharge begins above $109,000 for single filers and $218,000 for married couples filing jointly. Those figures show the risk; 2028 thresholds have not yet been announced. An inflation adjustment can push someone into a surcharge for the first time or increase an existing one.

How TIPS Create Taxable Income Without a Sale

TIPS principal changes with inflation, as measured by the CPI, which rose 3.7% in the year through September 2026. In a taxable brokerage account, that increase generally counts as taxable interest for that year, even though the extra principal stays in the bond. The IRS calls this original issue discount, or OID, and it is reported on Form 1099-OID.

He receives the adjusted principal when the bond matures, or he can sell sooner at market value. In the meantime, the amount reported as income raises his cost basis, which is the investment amount used to calculate taxable gain or loss. That prevents the same increase from being taxed again when he sells or when the bond matures.

A $10,000 Adjustment Can Trigger a $1,148 Surcharge

Social Security calculates the income-related monthly adjustment amount, or IRMAA, using modified adjusted gross income, meaning adjusted gross income plus tax-exempt interest. Taxable TIPS inflation adjustments are included in that figure as interest income. Social Security usually looks back two tax years, so 2026 income would generally determine the 2028 surcharge.

Suppose he files as a single taxpayer and has both Part B and Part D. Before the inflation adjustment, his Medicare-related income totals $105,000, including his pension, taxable Social Security benefits, IRA withdrawals and regular bond interest. The $10,000 adjustment raises that total to $115,000. Using 2026 brackets for illustration, that moves him past the first surcharge threshold.

At 2026 rates, crossing that threshold adds $1,148.40 a year in surcharges. IRMAA works like a cliff: being just $1 over $109,000 triggers the full first-tier charge. A married couple filing jointly faces the same cliff at $218,000, and each spouse on Medicare pays the surcharge.

The inflation adjustment by itself does not create grounds for relief through Form SSA-44. A separate qualifying event, such as retirement, reduced work hours or a spouse's death, may justify a request if it lowers the income Social Security uses. He can also ask for a correction if the agency relied on incorrect tax information. Simply receiving taxable income without cash does not qualify.

Inside an IRA, the Same Bonds Stay Off Your Return

Inside a traditional IRA or 401(k), the annual TIPS adjustment generally does not create current taxable income for the account owner. Taxable withdrawals can still raise Medicare income later, and required minimum distributions eventually limit flexibility over timing. Holding the bonds there delays the income; it does not necessarily eliminate the surcharge.

Series I savings bonds offer another way to control timing. Owners can generally defer reporting interest until redemption or final maturity, though they may choose annual reporting instead. Newly issued I bonds purchased in the most recent issuance window earn a 4.26% annualized composite rate for their first six months. Older I bonds can carry different rates.

Move Before Year-End to Lock in the Current Tax Picture

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