Introduction Although radioiodine therapy (RIT) has been used for the treatment of hyperthyroidism for many decades, there is no consensus on the optimal time of methimazole (MMI) discontinuation before RIT. The aim of this clinical trial is to study the effect of three different time points of MMI discontinuation on response to RIT. Method and patients Overall, 151 patients (18–65 years old), with Graves’ disease who were taking MMI and referred to I-131 therapy, were consecutively assigned to one of three groups, and MMI was discontinued for 24–48, 48.1–72, and 72.1–168 h in group, 1, 2, and 3, respectively. Radioiodine uptake was measured in all patients and the radioiodine dose was calculated according to the Quimby formula to deliver 7.4MBq of I-131 per gram of thyroid weight. Response to RIT was assessed at 1, 3, 6, and 12 months after RIT. Results A total of 102 women and 49 men were included in the study. The mean administered dose of I-131 was 362.9± 188.7MBq (9.8 ± 5.1 mCi) and the mean time to response for radioiodine was 4.1± 3.6 months. There was no significant difference between the three groups in age, thyroid weight, anti-TPO level, radioactive iodine uptake level, and radioiodine dose (P>0.1). Response to RIT at 1, 3, 6, and 12 months after administration was also not different between the three groups (P>0.57). Conclusion No difference was found in the response to treatment between patients with MMI discontinuation for 24–48, 48.1–72, and 72.1–96 h before RIT.

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