I have had hypothyroidism for close to 30 years. Let me tell you I was able to lose weight before my pregnancy after that, OUCH. It was not from lack of trying or dedication.
I get my levels checked every 3 months unless I "feel" lower, then more often. I have been to endocrinologists and it is not easy. I once gained 30 lbs in 3 months without changing my eating/exercise habits. (I went to the doctor's for an annual exam and then returned to show him my weight gain). My levels were way out. The pounds did not just melt off when my levels came back. In fact I am still struggling with that gain from years ago.
I had to switch off from Synthroid to another form of levothyroxine because my levels go out of whack. My doctor said that I am not his only patient with this problem.
The only thing that worked for me was NS last year. Yes it contains "garbage" and soy (not good for hypothyroidism) but it is the only thing that has worked longer term in 15 years.
Okay having said that, I am now experiencing pre-menopausal symptoms and had to go on hormones and guess what? I gained 25lbs in a short time period. This is why I am here.
It is frustrating, it is a pain but it is trial and error. It is hard for some who have not experienced thyroid problems to relate or believe but it IS real.
Hormonal imbalances affect people differently. Even if you can remain stable for years and lose weight, it may affect you later.
Sorry for the long post and good luck!