Potassium iodide therapy may not impair efficacy of radioiodine therapy for Graves’ hyperthyroidism if discontinued 2 weeks before administration of I-131

Objectives: Use of potassium iodide (KI) has been gaining popularity as an adjunctive to anti-thyroid drugs (ATDs) for treating hyperthyroidism in countries where dietary iodine intake is sufficient. The aim of this study was to examine whether 2 weeks’ withdrawal of KI therapy was appropriate in patients who are planned to receive radioactive iodine I-131 therapy (RAI) for Graves’ hyperthyroidism (GH).

Methods: Commercially available KI pill in our country contains 38mg of iodine per pill. 41 patients with GH (M 7/F 34. Average of age 43.6yrs.), who had been pretreated by a combination of ATD and KI or KI alone underwent RAI. Thyroid weight (TW:g) was determined using US or CT. Dose of KI (mg/day) ranged from 50 to 300S (average:173). KI was withdrawn 14 days prior to RAI while ATDs were withdrawn 3 to 4 days prior to RAI. Patients were instructed to follow a low iodine diet from 7 days before to 3 days after administration of I-131. Dose of I-131 was adjusted so that more than 100Gy was delivered to the thyroid. Otherwise, 555 MBq of I-131 was administered. As a parameter for total body iodine, urinary iodine concentration normalized by urinary creatinine (UIC: μg/gCRE) was measured before withdrawal of KI (UIC1) and on the day of RIA (UIC2). Thyroid function and TRAb(IU/ml) were determined on the day of RIA. 24hr-radioiodine uptake (RU; %) was determined using a gamma camera and I-123. All patients were followed up for more than 1 year after RAI. Outcome of RAI was defined as successful when either of euthyroidism, subclinical hypothyroidism, or hypothyroidism was achieved within a year after RAI. Results of UIC, RU, and efficacy of RAI in patients (KI group) were compared with those in 54 patients with GH, who were pretreated ATD alone before RAI (ATD group).

Results: The average value of TW(g), TRAb (normal range:less than 2 (IU/L) and FT4 (normal range: 0.82-1.70ng/dL) on the day of RAI were 63, 16.2 and 3.9 , respectively. The average value of UIC2(μg/gCRE) in KI group was decreased to less than 0.1% of UIC1 (119.0 vs. 172449.7), but was higher than that in ATD group (119.0 vs.82.2, p=0.010). However, difference in RU was not significant between KI group and ATD group (66.5 vs. 62.0, p=0.127). Additionally, successful rate of RAI in KI group was comparable to that in ATD group (82 vs.85%, p=0.724, OR=0.807) among patients with a thyroid weight less than 90 g.

Conclusions: Pretreatment by KI does not impair therapeutic efficacy of RAI for GH if the drug is discontinued 2 weeks before administration of I-131 and is combined with low iodine diet for one week . Thus, pretreatment with KI may be feasible for patients with GH who are to undergo RAI.