I think, better is to ween or stop your thyroid medicine

In my opinion your body is telling you to ween or stop the thyroid medicine, ... not to remove thyroid-medicine-side-effect with anti-acid .. just my humble comment. I think others will agree.
Sorry, I just noticed you are on meds; otherwise others/I may have told you that already.
Thyroid meds will make systems overwork, including stomach, if these are hormones; if these are immune suppressants, on the other hand, you would not want to have them either while advancing beyond day 5-7 into fasting.
DL