I have severe hypothyroidism. I was diagnosed about 8 years ago. Right before I was diagnosed I gained an extreme amount of weight (like 60 pounds), and I had always been a normal weight and athletic. Once my thyroid levels were normalized I was able to lose the weight and I've maintained my weight within 10 pounds or so ever since.
Thyroid medications work by increasing the T3 and T4 levels in your blood steam- so it will take several weeks for you to feel better. Then you'll have to get your blood levels tested a few times a year, but you WILL feel normal again!
The above comment about fish is confusing- fish is a great lean protein source, best eaten 2-3 times a week.