Evaluation of thyroid hormone levels and urinary iodine concentrations in Koreans based on the data from Korea National Health and Nutrition Examination Survey VI (2013 to 2015)

No nationwide data have been published about thyroid hormone levels and urinary iodine concentrations (UICs) in Korea. The Korea Centers for Disease Control and Prevention and the Korean Thyroid Association established a project to evaluate the nationwide thyroid hormone profile and UICs in healthy Koreans as part of the Korea National Health and Nutrition Examination Survey (KNHANES) VI (2013 to 2015), a nationwide, cross-sectional survey of the Korean population that enrolled 7,061 individuals who were weighted to represent the entire Korean population. Based on the KNHANES VI, the geometric mean value of serum thyroid stimulating hormone was 2.16 mIU/L, and its reference interval was 0.59 to 7.03 mIU/L. The mean value of serum free thyroxine was 1.25 ng/dL, and its reference interval was 0.92 to 1.60 ng/dL. The median UIC in the Korean population was reported to be 294 μg/L, corresponding to 'above requirements' iodine intake according to the World Health Organization recommendations. A U-shaped relationship of UIC with age was found. The prevalence of overt hyperthyroidism and overt hypothyroidism in the Korean population based on the KNHANES VI was 0.54% and 0.73%, respectively.