my hypothyroid story: I have Hashimotos thyroiditis, an autoimmune condition. I have been on the exact same dose of thyroid supplement (Synthroid) for 9 years. I have never yet needed an increasing amount of Synthroid.
I am not overweight, although one of the tipoffs that I had a thyroid problem was that I went from very underweight (and probably hyperthyroid) to gaining 20 lbs, and reaching a more "normal" weight, in the course of one year. I also had other hypothyroid symptoms.
My naturopath says that in the case of Hashimoto's, you really need to stay on the Synthroid until you can get your body to stop making anti-thyroid antibodies. If your body no longer produces anyti-thyroid antibodies, then you can safely and possibly successfully experiment with getting off synthetic hormone all together.
So that seems to be the scoop on autoimmune thyroiditis.
best of luck,