Bond Interest, CDs, I-Bonds, and IRMAA: How Fixed Income Affects Your Medicare Premiums

Retirees who shift to "conservative" fixed income often get an IRMAA surprise: Treasury bonds, CDs, and corporate bonds produce fully taxable ordinary income. Municipal bonds are federally tax-exempt — but Congress added muni interest back into the IRMAA MAGI calculation anyway. TIPS generate phantom income you haven't received in cash. Only I-bonds offer a genuine deferral opportunity. A retiree with $80,000 in base income who adds a $400,000 bond ladder generating $18,000/year in interest can inadvertently cross the IRMAA threshold — not this year, but two years later.

The IRMAA MAGI formula

IRMAA MAGI = your Adjusted Gross Income (Form 1040, Line 11) plus tax-exempt interest income (Form 1040, Line 2a). Nearly every type of fixed income affects one of those two components:²

• Taxable interest (CDs, Treasury notes, corporate bonds, TIPS OID) → flows into AGI on Schedule B, Line 1
• Municipal bond interest → reported on Form 1040, Line 2a, then added back explicitly to compute IRMAA MAGI
• I-bond redemption interest → flows into AGI on Schedule B in the year you cash the bond

How each fixed income type counts

Municipal bonds: the "tax-exempt" trap for IRMAA

Congress defined IRMAA MAGI specifically to include tax-exempt interest — the same definition used for the ACA premium tax credit. The logic: Congress wanted high earners to pay the full cost of Medicare Part B regardless of how their income was earned. A retiree holding $500,000 in a muni bond ladder at 3.6% generates $18,000/year in interest that pays zero federal income tax — but adds $18,000 directly to IRMAA MAGI.¹

The misconception is common: many high-income retirees assume that if income is "tax-exempt," it doesn't count for Medicare purposes. It does. Switching from a high-yield taxable bond fund to a muni bond fund reduces your federal income tax — but leaves your IRMAA exposure essentially unchanged.

TIPS phantom income: the OID you didn't receive

TIPS work by adjusting their principal with inflation each year. If you hold $100,000 in TIPS and the CPI rises 3%, your principal grows to $103,000 — but you only receive the coupon payment in cash. The $3,000 inflation adjustment is taxable ordinary income in the year it accrues, reported on a 1099-OID.³

This "phantom income" is real from the IRS's perspective. A TIPS investor with $200,000 in TIPS at a 3% inflation adjustment pays taxes on $6,000 of income they haven't received — and that $6,000 increases their IRMAA MAGI for the two-year look-back calculation. The fix is straightforward: hold TIPS in an IRA or 401(k), where the inflation adjustment compounds tax-deferred and has no current-year MAGI impact.

I-bonds: the deferral opportunity

Series I savings bonds are the one instrument in the fixed income universe that lets you choose when interest hits your MAGI. Cash-basis taxpayers — which covers most individuals — defer all interest until they redeem the bond or it matures at 30 years. No interest appears on your tax return in the years you hold.⁴

For IRMAA planning, this creates a genuine lever: hold your I-bonds through high-income years (peak work earnings, the year of a business sale, a Roth conversion year) and redeem them in a low-income year — typically a "Roth conversion window" year between ages 62–64 when you've retired but before full pension and Social Security income starts. All accumulated interest hits MAGI in that redemption year, but you can pick the year deliberately rather than having it distributed automatically each year the way CD interest is.

The limit: I-bond purchases are capped at $10,000 per person per year ($20,000/couple), so you can't hold an unlimited amount. For many retirees, I-bonds represent a meaningful allocation of high-quality fixed income, but not their entire bond portfolio.

CDs and Treasuries: ordinary income that stacks

A CD ladder is a common retirement strategy — buy CDs maturing at 3, 6, 9, 12, 18 months to generate predictable income. Each CD's interest is taxable ordinary income in the year it accrues, regardless of whether you roll the proceeds or spend them. A $300,000 CD ladder at 4.5% generates $13,500/year that flows directly into AGI and IRMAA MAGI.

Treasury bills have a slight advantage over CDs: the interest is exempt from state and local income taxes (where CDs typically aren't). But for IRMAA purposes, Treasury bill interest is treated identically to CD interest — fully ordinary income, fully counted.

Many retirees under-model how their bond portfolio interacts with other income. If you have $60,000 in pension, $32,000 in Social Security (85% taxable = $27,200), and $12,000 in RMDs, your base income is roughly $99,200 — only $9,800 under the $109,000 IRMAA threshold. A modest CD or Treasury ladder at $200,000 × 5% = $10,000/year crosses the line.

Worked example: Patricia's bond portfolio surprise

Adjusted scenario — Patricia redeems $30,000 in I-bonds in 2024: She bought I-bonds during 2021–2022 when rates were near 7%. Total deferred interest accumulated: $6,000. Adding that redemption interest pushes total IRMAA MAGI to $114,000 → Tier 1 → +$1,148/year in Medicare surcharges starting in 2026. Had she waited until a lower-income year to redeem, she'd stay in Tier 0.

2026 IRMAA brackets (based on 2024 MAGI)

2026 Part B and Part D IRMAA surcharges are based on your 2024 tax return.²

For married couples, both spouses pay the surcharge separately based on the household's combined MAGI. A couple at $280,000 combined pays +$1,148/year each — $2,296/year combined.

Calculate your fixed income IRMAA impact

Enter your income sources. The calculator shows your IRMAA MAGI, which tier you're in, and the annual Medicare surcharge.

5 strategies for fixed-income retirees

1. Move TIPS to a tax-deferred account

TIPS phantom income is unavoidable if you hold TIPS in a taxable brokerage account. The solution is asset location: hold TIPS inside a traditional IRA or 401(k), where the inflation adjustment compounds without generating current-year MAGI. When you eventually take distributions, those count as ordinary income — but you control the timing and amount. TIPS ETFs (like VAIPX or SCHP) work the same way and avoid the complexity of individual TIPS OID tracking.

2. Time I-bond redemptions to low-MAGI years

You bought I-bonds during 2021–2023 when rates were 6–9%. You don't need to redeem them now. The interest keeps accumulating tax-deferred until you cash them. Model your income trajectory: if you retire at 63 with a two-year Roth conversion window before pensions and Social Security start, that may be your best redemption window — you're deliberately generating income in those years anyway, and the I-bond interest can be layered in. Avoid redeeming in the same year as a large Roth conversion, a business sale, or a Roth 401(k) rollover.

3. Rethink the muni bond "IRMAA hedge" — it doesn't work

Swapping taxable bonds for muni bonds reduces your federal income tax — but has minimal effect on your IRMAA surcharges. Congress deliberately added muni interest back into IRMAA MAGI. If your goal is IRMAA reduction, munis are the wrong tool. Strategies that actually reduce IRMAA MAGI: QCDs (which reduce IRA distributions before they hit AGI), reducing RMDs via Roth conversions in earlier years, or buying TIPS inside an IRA instead of a taxable account.

4. Prefer broad bond ETFs over actively managed bond funds

Actively managed bond funds trade frequently to capture price movements, generating short-term and long-term capital gains that are distributed to shareholders at year-end — even if you didn't sell any shares. These distributions appear on your 1099-DIV and flow into IRMAA MAGI. Broad bond index ETFs (like BND or AGG) have dramatically lower portfolio turnover and rarely distribute capital gains. For taxable accounts, this matters: the same fixed income exposure with meaningfully less IRMAA exposure.

5. Project five years of fixed-income income before locking in a ladder

A 5-year CD ladder at today's rates might seem low-risk. But each rung's maturity generates taxable interest in that year — and in two years, that interest feeds into IRMAA surcharges two years later. Before locking in a large ladder, model: what does my total IRMAA MAGI look like in each of the next 5 years, stacking this interest on top of pension, SS, and RMDs? Some years (year of RMD onset, year Social Security starts, year of Roth conversion) are naturally high-income years where additional interest could cross a tier boundary. Build the ladder around those peaks, not into them.

Related guides

• What counts as MAGI for IRMAA — complete income breakdown with interactive calculator
• Capital gains and IRMAA — how investment sales affect Medicare premiums
• RMDs and Medicare premiums — required minimum distribution IRMAA interaction
• 7 strategies to reduce IRMAA surcharges — QCDs, Roth timing, capital gain management
• IRMAA bracket calculator — enter your full income and see your exact tier