Objectives of this study were to investigate the occurrence and aggressiveness of differentiated thyroid carcinoma (DTC) in patients with hyperthyroidism and to explore the influence of the changes in dietary iodine intake on the association of hyperthyroidism and DTC. Clinical records of 1,800 patients with DTC were reviewed. The characteristics of DTC cases with and without thyrotoxicosis and their thyroid tumors were comparatively assessed.In the current series, 76 patients (4.2%) presented with hyperthyroidism before thyroid surgery. The most common type of goiter among 76 patients was toxic multinodular goiter. Presence of lymph node metastasis and recurrence was only observed in patients with toxic diffuse goiter. When DTC cases with and without thyrotoxicosis were compared irrespective of the type of goiter, aggressiveness of thyroid carcinoma was not higher in the thyrotoxic group. Hyperthyroidism and thyroid carcinoma coexisted in 3.3% and 4.7% in cases diagnosed before and after national iodine supplementation program, respectively (P = 0.04). Clinical course of the disease was not different in the diagnosis of these cases. As a result of no significant difference in complete remission ratio, we conclude that there is no need for a different DTC treatment protocol in cases with coexisting hyperthyroidism.