Hi,
When I was hyperthyroid my legs were very weak, especially my thighs. My endo said this was due to the fact that an overactive thyroid causes some muscle degeneration (or atrophy was the way she put it), and so it makes your muscles (especially upper arms and thighs) very weak. Since I have had RAI and am now hypo and on Synthroid, I notice that my muscles are slowing but surely becoming stronger again. Hope this helps.
Kim