Hi thanks for your query
As per discussion you have low t3 and normal TSH ANDT4
TSH causes the thyroid gland to make two hormones: triiodothyronine
(T3) andthyroxine (T4). T3 and T4 help control your body's metabolism.
(TSH)-Evaluates overall thyroid function.
Free Thyroxine (T4)-Evaluates the amount of T4 available to the cells and tissues.
Free Tri -iodothyronine (T3)-Measures the amount of T3 (the active form of the hormone) available to the cells and tissues
Your weight gain should be due to low t3 ang you get benefited by treatment. Diet plan
Your doctor should exclude secondary hypothyroidism
(caused by pituitary disease
Take care get well soon