You don’t mention your thyroid hormone levels ~ it’s possible that your synthroid is being given in too high a dose, which would render you hyperthyroid and weight/strength loss issues would result.
Please do not ever stop taking synthroid ~ the RAI destroyed your thyroid, and your body must have thyroid hormone to survive. That’s what the synthroid provides. A higher/lower dose may be required, but please do not ever stop taking it (or at least some form of synthetic thyroid hormone replacement).