Purpose: This study aimed to establish the reference range of serum thyroid stimulating hormone (TSH) and free T4 (fT4) in Korean children and adolescents. We also evaluate the association of TSH and fT4 level according to age, sex, thyroid peroxidase antibodies (TPOAb), obesity, and socioeconomic status (SES). Methods: We analyzed data from 1,021subjects (537 male) aged 10 to 19 who underwent thyroid function test including TPOAb in the Korea National Health and Nutrition Examination Survey VI (2013-2015). Results: The reference interval of serum TSH and fT4 were 0.70 to 7.03mIU/L and 1.00 to 1.63 ng/mL. Based on these references, the prevalence of subclinical hypothyroidism was 1.27%. Positive TPOAb were found in 2.7% of subjects (males 1.9%, females 3.7%). TSH is negatively correlated with age. Free T4 is positively correlated with age only in male. Thus, fT4 was higher in male than female. There were no significant differences of TSH and fT4 levels according to obesity status, SES and positive TPOAb status. Only, subject in 4th quartiles (UI/Cr＞ 595 μg/g) showed higher mean TSH than others. Conclusion: The serum TSH and fT4 reference levels in the Korean teenagers based on healthy population were established. This study provides important baseline information for understanding patterns of thyroid dysfunction in children and adolescents.