Yes, you probably need to file an amended return. If you didn't include your 1099-SA on your return, then you wouldn't have reported the amount of distributions from your HSA that were for qualified medical expenses. The information reported to you on your 1099-SA is also reported to the IRS. The IRS matches up all information reported to them with what taxpayers report on their tax returns. Since you didn't include your 1099-SA they will propose making an adjustment to your return that adds the distribution, but not the amount of it that was spent on qualified medical expenses. They will send you a letter and proposed additional taxes, penalties, and interest. If any of the payments/purchases you made with your debit card were not for qualified medical expenses then those amounts are taxable and subject to a 10% penalty. You can wait for them to do that (it could be more than a year) or you can resolve the issue now by filing an Amended Return.
Yes, you probably need to file an amended return. If you didn't include your 1099-SA on your return, then you wouldn't have reported the amount of distributions from your HSA that were for qualified medical expenses. The information reported to you on your 1099-SA is also reported to the IRS. The IRS matches up all information reported to them with what taxpayers report on their tax returns. Since you didn't include your 1099-SA they will propose making an adjustment to your return that adds the distribution, but not the amount of it that was spent on qualified medical expenses. They will send you a letter and proposed additional taxes, penalties, and interest. If any of the payments/purchases you made with your debit card were not for qualified medical expenses then those amounts are taxable and subject to a 10% penalty. You can wait for them to do that (it could be more than a year) or you can resolve the issue now by filing an Amended Return.
What do you do if you already received the letter from the IRS with proposed changes accounting for the missed distribution from your HSA? Can I just send them a letter with the 1099SA that I forgot showing it was all for medical related expenses?
To correct a return that you have not received a letter from the IRS for, you would Amend Your Return and include a statement with any explanations needed for the changes.
If you received a letter from the IRS you would need to contact the IRS at the number on the letter to discuss how they would like you to proceed. Some things can be handled over the phone, if they need supporting documentation or something else, they will let you know and tell you how to and where to send it.