Your "most recent" Federal income tax return is usually used by the Social Security Administration (SSA) to determine whether you are subject to an income-related monthly adjustment amount or IRMAA for your Medicare Part B and Medicare Part D coverage.

This means the SSA bases your IRMAA on reported modified adjusted gross income (MAGI) from two years ago.   

For example, your 2025 Part B and Part D IRMAA is determined based on your 2023 tax return that was filed in 2024.

However, in some cases the SSA may determine your current IRMAA using earlier tax returns --- for instance, if the IRS only provided the SSA with your 2022 federal tax returns that you filed in 2023.

The Social Security Administration adds more detail with:

To determine your 20[25] income-related monthly adjustment amounts, we [Social Security] use your most recent federal tax return the IRS provides to us. Generally, this information is from a tax return filed in 20[24]for tax year 20[23].

Sometimes, the IRS only provides information from a return filed in 20[23] for tax year 20[22].  If we use the 20[22] tax year data, and you filed a return for tax year 20[23] or did not need to file a tax return for tax year 20[24], call us or visit any local Social Security office. We’ll update our records.

If you amended your tax return, and it changes the income we count to determine the income-related monthly adjustment amounts, let us know. Social Security needs to see a copy of the amended tax return you filed and your acknowledgment receipt from IRS. We’ll update our records with the information you provide, and correct or remove your income-related monthly adjustment amounts, as appropriate.

See the most recent IRMAA information in our Latest Medicare News: IRMAA information in our Latest Medicare News

The good news:  You can appeal your IRMAA assessment due to financial changes.

If your annual income is no longer over a specific IRMAA limit - and the SSA is using your older tax return (for example using a 2022 tax return filed in 2023 to determine 2025 IRMAA), even though you filed a 2024 tax return - or, due to financial changes - you were not required to file a 2024 tax return, you are able to contact a local SSA office and update your income records - and ask the SSA for a redetermination of your IRMAA - and possibly avoid your 2025 IRMAA payment.

As noted by the Social Security Administration:

"If your income has gone down due to any of the following situations and the change makes a difference in the income level considered by the Social Security Administration, contact the Social Security Administration and explain that you have new information and may need a new decision about your income-related monthly adjustment amount based on the following:

• You married, divorced, or became widowed;
• You or your spouse stopped working or reduced your work hours;
• You or your spouse lost income-producing property due to a disaster or other event beyond your control;
• You or your spouse experienced a scheduled cessation, termination, or reorganization of an employer’s pension plan; or
• You or your spouse received a settlement from an employer or former employer because of the employer’s closure, bankruptcy, or reorganization.

If any of the above applies to you, the Social Security Administration needs to see documentation verifying the event and the reduction in your income.

The documentation you provide should relate to the event and may include a death certificate, a letter from your employer about your retirement, or something similar.

If you filed a Federal income tax return for the year in question, you need to show the Social Security Administration your signed copy of the return."

Social Security has added the FAQ:

Question:  I am amending my tax return for previous years when I filed my tax return as single but I was in a same-sex marriage, can I have my IRMAA changed for those years too?

"If you amend your tax return and it changes the income we counted to determine the income-related monthly adjustment amounts, let us [Social Security] know. We [Social Security] need to see a copy of the amended tax return you filed and your acknowledgment receipt from IRS. We [Social Security] will update our records with the information you provide, and correct or remove your income-related monthly adjustment amounts, as appropriate."

(source: https://faq.ssa.gov/en-us/Topic/article/KA-02419) form: https://www.ssa.gov/forms/ssa-44-ext.pdf