PSAT264 Effectiveness of Thyroid Gland Size Determination by Ultrasonography versus Manual Palpation in Success of Radioactive Iodine Therapy for Graves’ Disease

Abstract 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 <20 g when the sizes were overestimated by palpation. 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 (<20 g) or large (>=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.