Objective: Thyrotoxicosis is a disorder of excessive thyroid hormone, whereas the term hyperthyroidism specifically describes increased thyroid hormone synthesis and secretion. Inorganic iodide has been used in combination with antithyroid drug (ATD) to achieve rapid normalization of thyroid hormone levels in severe thyrotoxicosis. There have been few reports on the effectiveness of potassium iodide (KI) for treatment of thyrotoxicosis in children. The purpose of the present study was to investigate the effectiveness of KI in children and adolescents with thyrotoxicosis. Methods: Sixty seven pediatric patients with newly diagnosed thyrotoxicosis due to GD at Ajou University Hospital from 2003 to 2012 were recruited. Among the sixty seven patients, 49 patients (73%) were treated with ATD alone while 18 patients (27%) were treated with combination of ATD and KI. 0.25cc of Lugol’s solution which is a saturated solution of KI, was administered in combination with ATDs three times a day during the first three days on admission. To evaluate the effectiveness of initial treatment, thyroid function tests (TFT) were repeated two weeks after the initiation of ATDs and TFTs together with measurement of thyroid antibodies were done eight weeks later. Results: No significant differences were found between serum levels of fT4, T3 and TSH at initial diagnosis, but mean fT4 values were significantly lower in the group receiving combined therapy of ATD + KI after two weeks of treatment (2.3 ± 0.6 vs. 2.8 ± 1.0, p 0.05). Similarly, serum T3 levels rapidly decreased after two weeks of treatment and mean levels were significantly lower in the group receiving combined therapy compared with those of the patients receiving ATD alone (200.8 ± 45.8 vs. 253.8 ± 117.5, p 0.05). However, serum levels of fT4, T3 and TSH showed no significant differences between two groups after eight weeks of treatment. Conclusion: In this study, more rapid reduction of thyroid hormones by combination therapy with ATDs and KI than with ATD monotherapy was observed in children and adolescents diagnosed as thyrotoxicosis due to GD. We suggest that the use of KI in combinations with ATD during the initial phase of treatment is beneficial in more effective normalization of thyroid hormones in childhood thyrotoxicosis.