The investigators used a dataset from the KNHANES VI (2013 to 2015), a nationwide, cross-sectional survey of the Korean population for surveillance of health and nutritional status. The survey used stratified, multistage clustered probability sampling to select a representative sample of the civilian non-institutionalized Korean population. Research subjects were selected through two-stage stratified cluster sampling using population and housing census data. The investigators included the subjects who were examined for TSH, FT4, TPO Ab, and UIC. Subjects previously diagnosed with benign thyroid diseases or thyroid cancer were excluded.