Abstract. Objective: Since the 1970s, low dose radioactive iodine-131 (RAI or I-131) has been widely reported in the treatment of patients with differentiated thyroid cancer (DTC). However, the clinical outcomes, dosage of I-131, and criteria for successful ablation differ in the various studies. The aim of this study was to assess the clinical outcome 18 months after RAI therapy in selected DTC patients, and to identify factors associated with a good response. Methods: In this experimental study, 105 subjects were randomly selected from patients with DTC referred to the Nuclear Medicine Department between December 2008 and April 2010 and who had indications for RAI therapy. Patients were randomly divided into 3 groups to receive an empiric low-dose therapy of either 30, 40, or 50 mCi of I-131. For 18 months after treatment, at 6-month intervals, patients were monitored closely clinically, with serum thyroglobulin assays, and with I-131 whole-body scans. Results: Among 105 patients who completed followup, 86 % were successfully ablated with a single low dose of I-131. There was no statistically-significant difference in ablation rates between the subgroups that receive 30,40, or 50 mCi of I-131. The cumulative ablation rate was 99% in patients after the second dose of low dose therapy. Conclusion: If appropriate selection criteria are used in DTC, successful remnant ablation can be achieved with low doses of I-131 in the range of 30-to-50 mCi. No significant differences were found in results achieved with 30, 40, or 50 mCi of I-131I. As the majority of the DTC patients fall within the inclusion criteria of this study, they can be treated on an ambulatory basis with associated low cost, convenience, and a low whole-body radiation-absorbed dose of I-131.

Keywords • Hyperthyroidism • Graves’ disease • Radioiodine therapy • Outcome • Hypothyroidism