Once we get heat wave here, I am not doing so good. yes, salt helps. I am taking one flat tea spoon in the morning of Sea Salt from local grocery store ( I don't see difference between expensive Celtic salt and this one ). Another thing is ZMA. We are losing lots of magnesium through sweat. Did you do iron studies? Not just ferritin but TIBC, serum iron, etc.? I have low normal iron. Once I started taking iron sup, I am feeling better. In this topic, there are several posts about iron importance. I posted a link on FT3. there is explanation that thyroid does not work without iron, etc.
Heat intolerance is generally due to thyroid issues. I never had this problem before AF happen.