Pediatric reference intervals for TSH, FT3 and FT4 and the relevance of BMI and puberty in measurement interpretation.

BACKGROUND The present study aimed to establish age- and sex-specific reference intervals for serum concentrations of thyroid-stimulating hormone (TSH), free triiodothyronine (FT3) and free thyroxine (FT4) in healthy children and adolescents. Additionally, we investigated the association of TSH, FT3, and FT4 with putative influencing factors such as sex, body mass index (BMI) and puberty. METHODS A total of 9,404 blood serum samples from 3,140 children and adolescents without thyroid affecting diseases were included in determining TSH, FT3 and FT4 levels and age- and sex-specific reference ranges. To investigate the association of TSH, FT3 and FT4 with age, sex, weight status and the role of puberty-based changes, the hormone levels and BMI values were converted to standard deviation scores (SDS). RESULTS In general, TSH, FT3 and FT4 were found to be age- and sex-dependent. Puberty was accompanied by decreased TSH, decreased FT3 with a temporary peak in males and a temporary nadir of FT4 in Tanner 3 for both sexes. BMI-SDS was positively associated with TSH-SDS (s=0.081, p 0.05). BMI-SDS was positively associated with FT3-SDS (s= 0.066, p 0.05). CONCLUSIONS Age- and sex-specific reference intervals are important for the interpretation of measurements of TSH, FT3 and FT4 in children and adolescents. Influencing factors such as BMI and puberty should be taken into consideration when using measurements of TSH and thyroid hormones in the diagnosis, treatment and monitoring of thyroid diseases.