Hi Suzy,

TSH is a pituitary hormone that normally helps regulate thyroid hormone levels. After RAI, we don't need TSH for thyroid hormone production because we don't have many follicular thyroid cells. Also, our pituitary gland recognizes TSH receptor antibodies in the blood as if they were TSH. These antibodies increase dramatically after RAI. Thinking we have enough TSH when we have antibodies instead, the pituitary stops secreting TSH. So TSH levels are falsely decreased after RAI and results are misleading. There is no separate reference range for TSH in post RAI patients. However, many doctors realize the TSH is misleading in this patient population and don't run this level.

FT4 and FT3 are measurements of your available thyroid hormone. These levels tell if you have enough or too much of these hormones and they determine if your dose of replacement hormone is appropriate. With FT4 and FT3 you can tell the right type and amount of replacement hormone. By 11 years after RAI, most patients need to be on both T4 and T3 replacement hormone. Best, Elaine