Weight gain is a symptom of hypothyroidism and the fact that you're still gaining/not losing is indicative of insufficient thyroid hormones.
What are your current thyroid hormone levels? You should be getting tested periodically, for TSH, Free T3 and Free T4. Do you know if you have Hashimoto's Thyroiditis? With Hashimoto's, thyroid hormone output is steadily declining, due to damage from the antibodies, until it will eventually produce no hormones on its own and you will be completely dependent on replacement hormones. With Hashimoto's, medication must be periodically, increased to accommodate the lack of thyroid activity.
MH has a very active Thyroid Disorders community, which can be accessed via the following link:
http://www.medhelp.org/forums/Thyroid-Disorders/show/73
All of that said, losing weight while hypo is difficult, at best and seemingly impossible, but can be done if you're very diligent. You'll need to focus on lots of fresh (or frozen) veggies, lean protein, good fats, etc. Exercise is very important, to build muscles, which burn fat.
