Excessive Iodine Intake and Subclinical Hypothyroidism in Children and Adolescents Aged 6-19 Years: Results of the Sixth Korean National Health and Nutrition Examination Survey, 2013-2015

Background: Iodine is an important element for the synthesis of thyroid hormone, and its deficiency or excessive intake is associated with various thyroid diseases. Little is known about the association between iodine status and thyroid function among children and adolescents living in iodine-rich areas. Therefore, this study analyzed this association using data from a nationwide survey. Methods: From the Korea National Health and Nutrition Examination Surveys VI (2013-2015) data, 1288 subjects (711 male) aged 6-19 years who underwent a urinary iodine concentration (UIC) test and 1000 subjects (564 male) aged 10-19 years who underwent a thyroid function test were included in this study. Serum levels of thyrotropin (TSH), free thyroxine (fT4), and thyroperoxidase antibodies (TPOAb) were analyzed. Subclinical hypothyroidism (SCH) was defined as TSH>5.5 mu IU/mL with a normal fT4 level. Median daily iodine intake was calculated from the UIC. Daily sodium intake was derived from the nutritional survey data of 1181 subjects. Results: The median UIC was 449 mu g/L (range 15-21,905 mu g/L), and the prevalence rates of UIC >= 300 mu g/L and >= 1000 mu g/L were 64.9% and 25.0%, respectively. The prevalence rates of a sodium intake >2000 mg/day and iodine intake >2400 mu g/day were 75.0% (885/1181) and 12.7% (164/1288), respectively. The prevalence rates of SCH and TPOAb >34 IU/mL were 7.2% (72/1000) and 2.3% (23/1000), respectively. The prevalence of SCH was significantly higher in the iodine deficient and iodine excess groups compared to those in the UIC 100-299.9 mu g/L group (p = 0.038). Therefore, there was a U-shaped and inverted U-shaped correlation between serum levels of TSH and fT4 with UIC, respectively. These correlations were especially prominent when UIC was >1000 mu g/L. There was no definite correlation between TPOAb and UIC. Conclusions: Excess iodine was prevalent in Korean children and adolescents, and it may be associated with SCH. Therefore, monitoring the iodine status and education on adequate intake are needed in iodine-rich areas.