something similar happened to me and it turned out to be Graves Disease (sweating, losing my breath, loss of hair, brittle nails, shaking hands, weak legs). I gained weight, a lot of it, and while it was associated to my thyroid, no one could understand why I gained when I should have been losing. I went to Dr. Feinstein at Medical City (info below).
I'm still trying to work on the weight loss but to be quite honest, I haven't really taken the time to focus on losing it. I did get tons of blood work done and I was just told that I would have to work out extra hard and lower the amount of sugar I take in because my metabolism has significantly slowed down and that my body doesn't process sugar as quickly as it should/did. Age is a wonderful thing, but it comes along with it's quirks :)
Good luck to you!
David Feinstein, M.D.
Endocrinologist
Medical City Dallas
7777 Forest Lane, C-604
Dallas, TX 75230
Office: ###-###-####
Fax: ###-###-####)