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Background Thyroid gland size is a required parameter for radioactive iodine (RAI) dose calculation for radioactive iodine therapy (RAIT) of Graves’ disease. Comparing between ultrasonography and palpation, previous studies showed that the former provided more accurate thyroid sizes. However, there have been no studies comparing success of RAIT when either of these two methods is used to determine thyroid gland size. The aim of this study is to evaluate the outcome of RAIT using thyroid gland size measured by ultrasonography compared with palpation for RAI dose calculation. Methods We prospectively enrolled Graves’ disease patients who presented for the first RAIT. Thyroid gland sizes of all participants were determined by both ultrasonography and palpation on the day of RAIT. Participants were then randomized into ultrasound (US) or palpation groups for which RAI dose administered was calculated using the thyroid size obtained by the method assigned to their groups. Primary outcome was success of RAIT at 6 months, defined by the ability to discontinue antithyroid drugs (ATDs). Secondary outcomes were factors associated with success of RAIT. Results One hundred patients completed the study, 52% were in the US group and 42% were in the palpation group. Success rate of RAIT at six months was higher in the palpation group (58.3% vs 38.5%, p = 0.047). Among patients who had successful RAIT, the palpation group had a higher rate of hypothyroidism than did US group (78.6% vs 60%) as well as higher RAI dose (9.5 ± 3.6 millicuries in palpation group vs 7.1 ± 4.8 millicuries in US group, p = 0.005). There were no factors associated with favorable outcome, including sex; age; weight; type or dose of previous ATDs; duration of disease; baseline thyroid function tests; baseline anti-TSH receptor antibody levels and urine iodine. There was a poor correlation between thyroid sizes, obtained by ultrasonography and palpation (correlation coefficient = 0.578). The least discrepancy in size was observed when thyroid glands were between 40 and 60 grams (g). The highest difference was seen in patients with thyroid sizes Conclusion Success of RAIT was higher in palpation group than in US group. Overestimation of thyroid gland size accounted for the higher RAI dose and success of palpation group at the expense of more hypothyroidism. Low constant value per gram of thyroid tissue in the calculation formula might underlie the overall low success rate. Palpation tended to incorrectly estimate the size when thyroid gland is either small (=60 g). Therefore, adequate constant value in the calculation formula and obtaining thyroid gland size by ultrasonography prior to RAIT in patients with extreme thyroid sizes may be beneficial. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.

Background. The long-term continuation of the low-dose antithyroid drug (ATD) beyond the standard duration of ATD therapy of 12–18 months to prevent recurrent hyperthyroidism (RH) is recommended with low quality of evidence. Objectives. To examine whether long-term continuation of low-dose ATD beyond the recommended duration of treatment would provide a benefit in the prevention of RH in patients with Graves’ hyperthyroidism (GH) who achieved euthyroid status with a standard course of ATD therapy. Methods. A 36-month prospective randomized controlled study was conducted in 184 patients who had first diagnosed GH and were treated with a standard regimen of ATD therapy using methimazole (MMI) until achieving euthyroidism that was stably maintained for at least 6 months with a low-dose of (2.5–5 mg/day) MMI. All patients had neither a history of adverse effects from MMI, recurrent GH, severe and active ophthalmopathy nor conditions known to affect thyroid function before randomization. The patients were randomized into 2 groups: one group (92 cases) was assigned to discontinue (DISCONT-MMI) and the other (92 cases) was assigned to continue low-dose MMI (CONT-MMI) that was taken at the time of enrollment. The patients in both groups were followed up at 3, 6, 12, 18, 24, 30, and 36 months. The rate of RH was compared between both groups, and the adverse effects and risk factors of RH were also studied. Results. At the end of the 36-month study, 83 cases in CONT-MMI and 90 cases in DISCONT-MMI were eligible for analysis. The cumulative rates of RH in CONT-MMI were significantly lower than those in DISCONT-MMI at every follow-up time point (1.2% vs. 11.2%, 6.8% vs. 18.4%, 11.0% vs. 27.2%, 11.0% vs. 35.0%, and 11.0% vs. 41.2% at 6, 12, 18, 24, and 36 months, respectively; p < 0.01 ). Cox proportional hazard multivariate analysis showed that there were 2 factors independently associated with the risk of RH, including continuation of low-dose MMI therapy, which decreased the risk of RH by 3.8 times (HR = 0.26, p = 0.007 , 95% CI = 0.10–0.70) and age onset of hyperthyroidism before 40 years, which increased the risk of RH by 2.9 times (HR = 2.9, p = 0.015 , 95% CI = 1.23–6.88). Neither minor nor major adverse effects of low-dose MMI therapy were observed during the study period. Conclusions. In Graves’ hyperthyroid patients with no or nonsevere ophthalmopathy who have completed a standard course of methimazole therapy without an adverse effect and have achieved an euthyroid status that is stably maintained with low-dose methimazole, a long-term continuation of the low-dose methimazole of 2.5–5 mg daily is effective and safe in the prevention of recurrent hyperthyroidism or maintenance of euthyroid status as long as the low-dose methimazole is continued. (TCTR20170705002).