Purpose: The optimal activity of radioiodine (I-131) administered for ablation therapy in papillary and follicular thyroid cancer after thyroidectomy remains unknown in a long-term (> 10 year) follow-up. Some, shorter follow-up studies suggest that activities 1.1 GBq and 3.7 GBq are equally effective. We evaluated the long-term outcomes after radioiodine treatment to extend current knowledge about the optimal ablative dose of I-131.Methods: One hundred and sixty consecutive adult patients (129 females, 31 males; mean age 46 ± 14 y, range 18-89 y) diagnosed with histologically confirmed differentiated thyroid cancer, were randomised in a prospective, phase III, open-label, single-centre study, to receive either 1.1 GBq or 3.7 GBq of I-131 after thyroidectomy. At randomisation, patients were stratified according to the histologically verified cervical lymph node status and were prepared for ablation using thyroid hormone withdrawal. No uptake in the whole-body scan with I-131 and serum thyroglobulin concentration less than 1 ng/mL at 4-8 months after treatment was considered successful ablation.Results: Median follow-up time was 13.0 years (mean 11.0 ± 4.8 y; range 0.3-17.1 y). Altogether 81 patients received 1.1 GBq with successful ablation in 45 (56%) patients. In the original study, thirty-six patients (44%) needed one or more extra administrations to replete the ablation. Of these, 4 (8.9%) and 5 (14%) patients relapsed during the follow-up, respectively. Of the 79 patients treated with 3.7 GBq 45 (57%) had successful ablation after one administration of radioiodine and 34 (43%) needed several treatments. Of these, 2 (4.4%) and 9 (26.5%) patients relapsed, respectively. The groups did not differ in the proportion of patients relapsing (p = .591).Conclusion: During follow-up of median 13 years, 3.7 GBq is not superior to 1.1 GBq in the radioiodine treatment after thyroidectomy in papillary and follicular thyroid cancer.