Long-term outcomes after a radioactive iodine treatment for a single autonomous functioning thyroid nodule in Japan.

We investigated the association between a 500 MBq dose of radioactive iodine treatment (RAIT) and both thyroid nodule volume and thyroid function in patients with a single autonomous functioning thyroid nodule (AFTN). We retrospectively studied 201 patients with an AFTN who received RAIT at a dose of 500 MBq and were followed up for more than 2 years. Thyroid function at diagnosis, thyroid antibody positivity, treatment with antithyroid drugs before RAIT, cystic components of the nodule, and ¹³¹I uptake outside the nodule were assessed. Nodule enlargement was observed in 18 patients (9%), persistent hyperthyroidism in 13 patients (6.5%), and hypothyroidism in 45 patients (22.3%). Nodule volume before RAIT was significantly larger in the nodule enlargement group compared to the non-enlargement group. The risk factors for persistent hyperthyroidism were larger nodule volume and absence of a cystic component in multivariate analysis. The cutoff nodule volume before RAIT for predicting nodule enlargement was 15.5 mL, and for predicting persistent hyperthyroidism was 16.6 mL. Nodule volume decreased to 47% in the first year and continued to gradually decrease thereafter. This study provided long-term outcome data regarding nodule volume change and thyroid function in AFTN patients following single fixed-dose RAIT, and it identified risk factors for nodule enlargement and persistent hyperthyroidism after RAIT. Nodule volume before treatment was a good predictor of treatment response.