Circulating thyroid hormone levels in children.

Extensive use of radioimmunoassay for routine measurement of serum thyroid hormones in paediatric thyroid disorders showed inconsistencies between laboratory results based upon adult criteria and clinical observation. To resolve this disparity, serum triiodothyronine (T3) and thyroxine (T4) levels were measured by radioimmunoassay in 354 healthy children aged between 3 weeks and 17 years. The mean serum T3 concentration in children up to 10 years of age was 1·94±0·35 ng/ml (SD) which was higher than the mean serum T3 of 1·37±0·25 ng/ml in healthy adults. Similarly, the mean serum T4 of 10±2·5 μg/100 ml was higher than the adult mean serum T4 of 8·5±1·5 μg/100 ml. Neither concentration changed significantly from 3 weeks to 10 years of age, nor was there any sex difference. In girls serum T3 and T4 concentrations declined gradually from age 10 to maturity. A perimenarcheal nadir observed in the T4 data was thought to reflect the joint effects of the age-dependent fall in circulating T4 and the concomitant oestrogen-dependent rise in thyroxine-binding globulin. In boys the decline in serum T3 occurred approximately 2 years later than in the girls. These observations show that the normal ranges for serum T3 and T4 in children are higher than those in adults and that reference to normal adult ranges may lead to misclassification in diagnosis and monitoring of paediatric thyroid disorders.