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