Reference intervals for TSH and thyroid hormones are mainly affected by age, body mass index and number of blood leucocytes, but hardly by gender and thyroid autoantibodies during the first decades of life

Objectives The purpose of our study was to establish reference intervals for thyroid function tests in children and adolescents and to identify factors that may influence the limits of these intervals. Methods TSH, FT3, FT4, T3, T4, t-uptake, TPO-antibody (TPO-Ab) and TG-antibody (TG-Ab) levels were determined in blood of 1004 infants, children and adolescents by the Elecsys system (Roche). Results A distinct overall age-dependent decrease of analyte levels was found for all parameters investigated. Puberty was accompanied by an increase of TSH, FT3 and T3 levels. Results of T4 and t-uptake were significantly higher in girls compared to boys. The exclusion of children with increased TPO-Ab and TG-Ab had no significant effect on the limits of the reference interval. We found that besides age, BMI-SDS but also white blood cells count and gender played a role in the prediction of analyte variation. Conclusions Covariates like BMI-SDS and white blood cell count should be taken into consideration when interpreting TSH and thyroid hormone measurements as well whereas gender and TPO-Ab or TG-Ab play a minor role.