Outcome of radioiodine therapy for Graves' disease pretreated with potassium iodide: A comparison with the pretreatment with methimazole

We retrospectively reviewed the outcomes of radioiodine therapy (RIT) in patients with Graves’ disease (thyroid volume: 10.0–79.9 mL) pretreated with either potassium iodide (KI) or methimazole (MMI). ¹³¹I was administered at a fixed dose of 481 MBq. KI was discontinued for 3 days (KI3 patients, n = 159) or 5 days (KI5 patients, n = 174), MMI was discontinued for 2 days (MMI patients, n = 472) before RIT, and KI and MMI were resumed two days after RIT. The pre-RIT thyroid volumes were a median 27.8 mL, 30.4 mL, and 34.6 mL, and the overall remission (no treatment with KI and MMI after obtaining an improvement in the thyroid function) rates at 1 year after RIT were 89.9 %, 93.1 %, and 80.0 % in KI3, KI5, and MMI patients, respectively. The remission rates classified by thyroid volume were largely equivalent among patients with KI3, KI5, and MMI. KI3 patients with a thyroid volume of <50.0 mL and KI5 patients with a thyroid volume of <20.0 mL had significantly lower thyroidal ^99mTc uptake rate/thyroid volume on the day of RIT in comparison to MMI patients. In RIT using fixed 481 MBq of ¹³¹I, KI pretreatment did not negatively affect the outcome of RIT. KI can be discontinued three days before RIT in patients with severe hyperthyroidism. KI should be discontinued at least five days before RIT in patients with mild hyperthyroidism, especially in those with a small goiter, to enhance the avidity of the thyroidal ¹³¹I uptake.