Can your thyroid return to normal after radiation treatment?

No. If it is radiation to the thyroid, it never becomes normal and need to have life long thyroid supplementation.
YES. However, that's not usually the desired result. Hyperthyroid patients who receive radioiodine treatment are best off if the gland stops working for a couple of reasons: 1) grave's disease could relapse. 2) exposure to radioactive iodine increases your risk for later developing thyroid cancer as long as your gland is active.