We aimed to determine the factors predicting hypothyroidism after radioactive iodine (RAI) treatment in patients with toxic adenoma and toxic multinodular goiter.

We retrospectively collected the data of 237 patients with toxic multinodular goiter or toxic adenoma who had consecutively received RAI treatment between 2014 and 2020 at 2 medical centers. Patients who received the second RAI treatment and whose medical records could not be accessed were excluded from the study. Finally, 133 patients were included in the study. RAI was administered at an empirical dose of 15 or 20 mCi.

The median age of the 133 participants was 69 years (interquartile range, 62-75 years), and 64.7% of the participants were women. A total of 42.1% of the patients had toxic adenoma, whereas 57.9% of patients had toxic multinodular goiter. The median follow-up was 24 months (interquartile range, 11-38 months). During the follow-up, 61.7% of patients became euthyroid, 30.8% developed hypothyroidism, and 7.5% remained hyperthyroid. The median month of hypothyroidism onset was 4 months (interquartile range, 2-9 months). Regression analysis revealed 2 factors that could predict hypothyroidism: thyroid-stimulating hormone (odds ratio, 2.548; 95% CI, 1.042-6.231; P = .04) and thyroid volume (odds ratio, 0.930; 95% CI, 0.885-0.978; P = .005).

Overall, 30.8% of the cases developed hypothyroidism after the RAI treatment. Approximately 78% of hypothyroidism developed within the first 10 months. The risk of hypothyroidism was higher in patients with higher thyroid-stimulating hormone and smaller thyroid volume.