My wife takes synthroid for Hashimoto’s, which causes hypothyroidism. It didn’t affect her weight (she was thin anyway), but it did warm her up and helps her mind stay focused.
I tried some thyriod pills for a few days without doctor’s approval. I felt great at first, but after a couple days I was super-hot and even fatigued. It wasn’t fun at all. Experimenting with thyroid hormones, when your lab numbers are all normal, doesn’t sound like a good idea to me.