Reversing thyroid treatment?

I never had a problem with my weight until I had treatment for an over-active thyroid gland. I had radioactive iodine which killed it dead. At the time they went through the side effects but never mentioned that I'd put on nearly 2 stone in a month but that's what happened. Yes I know the best way to lose weight is exercise and healthy eating and that's what I do but I can't get it shifted. Does anyone know if it is possible to kick start your thyroid working again??