Korean pediatric reference intervals for FT4, TSH, and TPO Ab and the prevalence of thyroid dysfunction: A population-based study

Background: The appropriate age-related reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) are important in interpreting the results of thyroid function tests in children and adolescents. However, these reference intervals are scanty. This study aimed to establish the reference intervals for FT4, TSH, and thyroid peroxidase antibody (TPO Ab), and identify the epidemiological prevalence of thyroid dysfunction in a Korean pediatric population. Methods: The data from a sample of 2042 children and adolescents aged 10-18 years who underwent a nationwide Korean representative sampling and cross-sectional survey with blood collection for a thyroid function test using an electrochemiluminescence immunoassay were evaluated. Results: Based on the National Academy of Clinical Biochemistry (NACB) criteria, the median and range (2.5th - 97.5th percentile) for FT4 and TSH in total were 16.47 pmol/L (12.61-21.49 pmol/L) and 2.48 mIU/L (0.63-7.03 mIU/L), respectively. Positive TPO Ab (3.1%) by NACB was > 19.16 kIU/L (boys) and > 29.30 kIU/L (girls) for adolescents aged 10-18 years. FT4 and TSH were associated with increasing and decreasing age, respectively. Boys' FT4 and girls' TPO Ab levels were greater than for the other sex, respectively. Using the new reference interval, overt hypothyroidism, subclinical hypothyroidism, subclinical hyperthyroidism, and overt hyperthyroidism were found in 0.5%, 2.1%, 1.3%, and 1.2%, respectively, of Korean children and adolescents. Conclusions: This study provides evidence-based age-and sex-specific reference intervals for thyroid hormones and autoantibodies, and identifies the current prevalence of thyroid dysfunction in a Korean pediatric group.