Maybe take a salt tablet and watch your water intake during the day. Just a wild guess on my part.
Are salt tablets different to just salt? I sometimes put a tiny bit of salt into my water, but just to compensate for lost salt. I picked it up in Sicily, where they sell water, lemon juice and a pinch of salt near the beach and it’s an amazing pick-me-up.
Thanks, Filmore, but I have actually had it checked a few times. No thyroid problems, I’m just sweaty and best suited to live in cold climates. Humidity is my enemy… perhaps I am part polar bear. 
I, too, have normal thyroid function. It’s one the tests my doctor runs most years.
I’ve been sweating excessively outside lately here in Virginia. I think it’s a combination of very high humidity and high UV from the sun.
I used to sweat like a pig, while everyone else was shivering in a sweater. Then I became diabetic, and now my body’s thermostat is normal. Not sure of the cause and effect, though.