Excessive Iodine Intake and Thyrotropin Reference Interval: Data from the Korean National Health and Nutrition Examination Survey

Background: Iodine intake is associated with various thyroid diseases and serum thyrotropin (TSH) levels. The aim of this study was to investigate iodine intake and its impact on the distribution of serum TSH levels using nationwide data from Korea, a country known to be iodine replete. Methods: The sixth Korean National Health and Nutrition Examination Survey (2013–2015) is a nationwide, cross-sectional survey of the Korean general population. Participants were selected using two-stage stratified cluster sampling of the population and housing census data. A total of 6564 participants aged ≥10 years who underwent thyroid function tests and urinary iodine level measurements during the survey were included in this study. Results: The median urinary iodine concentration (UIC) was 299.3 μg/L (interquartile range 158.8–699.8), suggesting more than adequate iodine intake in Korea. With high-iodine intake in all age groups and in both females and males, the TSH reference interval in the Korean reference population was right-shifted at 0.62–6.84 mIU/L. The prevalence of overt and subclinical hypothyroidism in Korea according to this reference interval was 0.73% and 3.12%, respectively, and was significantly associated with nutritional iodine status (p = 0.011 and p < 0.001, respectively). Conclusions: This first nationwide survey demonstrates more than adequate iodine intake and a right-shifted distribution of serum TSH levels in the Korean general population. Nutritional iodine status is important for determining serum TSH levels and is associated with a high prevalence of hypothyroidism. Nutritional iodine status might need to be considered when establishing TSH reference intervals of populations in iodine-replete areas.