Background: Iodine is an important element for the thyroid hormone production and its deficiency or excessive intake is associated with various thyroid diseases. As there are little known about iodine status and thyroid function in children and adolescents living in iodine-rich area, therefore, we analyzed these concerns based on a nationwide survey. Methods: From the Korea National Health and Nutrition Examination Surveys (KNHANES) VI (2013-2015) data, 1288 subjects (711 male) aged 6-19 years old who underwent urinary iodine test and 1000 subjects (564 male) aged 10-19 years old who underwent thyroid function test were included in this study. Serum levels of TSH, free T4, and thyroperoxidase antibody (TPO-Ab) were analyzed. Daily iodine intake was calculated from urinary iodine concentration (UIC). Daily sodium intake was reviewed from nutritional survey data. Results: In total 1288 subjects, the median (ranges) of UIC were 449.4 (14.5-21904.9) µg/L. The prevalence of UIC ≥300 µg/L was 64.9% and UIC ≥1000 µg/L was 25.0%. The prevalence of sodium intake ＞2000 mg/day and iodine intake ＞2400 µg/day were 75.0% (885/1181) and 12.7% (164/1288), respectively. In 1000 subject, the prevalence of TSH ＞5.5 µIU/mL and TPO-Ab ＞34 IU/mL was 7.2% and 2.3%, respectively. The prevalence of TSH ＞5.5 µIU/mL was significantly higher in iodine deficiency and excessive groups compared to those of UIC 100-299.9 µg/L groups ( P=0.038), therefore, there was the U-shape and inverted U-shape correlations between serum levels of TSH and free T4 with UIC, respectively. Those correlation showed more prominent especially when UIC was ＞1000 µg/L. There was no definite correlation between TPO-Ab and UIC. Conclusions: Iodine excess was prevalent in Korean children and adolescents and this may result in subclinical hypothyroidism. Therefore, monitoring of iodine status and education for adequate intake is needed in iodine-excessive area.