Most likely your 2015 tax returns that were filed in 2016.  Your 2017 Income-Related Monthly Adjustment Amount (IRMAA) is based on the most recent tax return that the Internal Revenue Service (IRS) provides to the Social Security Administration (SSA) and in general, the IRS will report the financial information (modified adjusted gross income (MAGI)) provided on your 2015 federal tax returns that were filed in 2016.

However, in some cases the SSA may determine your 2017 IRMAA using earlier tax returns --- for instance, if the IRS only provided the SSA with 2014 federal tax returns that you filed in 2015.

The good news: If your annual income is no longer over a specific IRMAA limit - and the SSA is using your 2014 tax return, even though you filed a 2015 tax return - or, due to financial changes - you were not required to file a 2015 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 2017 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.

You can read more about the 2017 Medicare Part D and Medicare Part B IRMAA income limits here:
Roughly a 4.5% increase in the 2017 Income Related Monthly Adjustment Amounts (IRMAA) for Medicare beneficiaries with higher annual incomes.

If your IRMAA redetermination request is not granted or initially denied, you can appeal the SSA IRMAA determination.

For more information about appealing your IRMAA determination, please see: https://q1medicare.com/faq/FAQ.php?faq=What-can-I-do-if-I-do-not-agree-with-the-Social-Security-Administration-s--income-related-monthly-adjustment-amount-or-IRMAA-assessment-&faq_id=544