Duration of antithyroid drug treatment may predict weight gain after radioactive iodine therapy in patients with Graves’ disease

BACKGROUND: Weight gain post-radioiodine (RAI) treatment is observed in patients with hyperthyroid Graves’ disease. Previous studies, mostly in Caucasian patients, demonstrated excessive weight gain averaging 5–7 kg from initial presentation. AIM: The aim of this study was to determine the extent and risk factors of weight gain in Thai patients with RAI-treated Graves’ disease. METHODS: This was a 5-year retrospective study of patients with hyperthyroid Graves’ disease who received RAI treatment during 2016–2020. The proportion and associated risk factors of weight gain ≥5% in patients who was followed for at least 3 months when compared with weight at RAI administration were analyzed. RESULTS: There were 347 patients with Graves’ disease (females 81.0%, mean age 38.8 ± 12.1 years, BMI 23.3 ± 4.0 kg/m(2)) who were treated with RAI. Almost all RAI-treated patients (91.9%) eventually developed hypothyroidism. During the median follow-up period of 25 months, 73.1% of them had weight gain. The mean weight change was +2.5 ± 4.9 kgs when compared with weight at the time RAI administration and +3.4 ± 6.5 kgs when compared with recalled body weight before the onset of hyperthyroidism. The proportion of patient in the obesity class I (BMI 25.0–29.9 kg/m(2)) increased from 23.6% to 28.0% and obesity class II (BMI ≥30.0 kg/m(2)) increased from 5.2% to 8.9%. Duration of antithyroid drug treatment less than 6 months after the diagnosis of hyperthyroidism was the only factor associated with weight gain ≥5%. CONCLUSIONS: Weight gain post-RAI treatment was common, and a significant proportion of patients went on to develop obesity. Early intervention with weight management support should be employed in patients with less than 6 months of antithyroid drug treatment before RAI.