SUMMARY 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 1 7 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 ,tg/100 ml was higher than the adult mean serum T4 of 8-5±1 5 ,ug/l00 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 oestrogendependent 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.Simple and rapid radioimmunoassays for measurement of serum thyroxine (T4) and triiodothyronine (T3) are now readily available for assessment of thyroid function in children, yet the criteria for normal thyroid function are mostly derived from adult data (Fisher, 1973). Using a competitive protein binding assay, O'Halloran and Webster (1972) reported that after the neonatal surge in thyroid hormone concentration, serum T4 declined till the age of one year to stabilize at a mean level of 9-2 Fg/100 ml. Murray et al. (1971) showed higher serum T4 and T3 resin uptake levels in childhood which declined slowly with age until late adolescence. Published information on circulating T3 levels is, however, sparse and conflicting. AvRuskin et al. (1973) found no significant change in six groups of children aged between 1 and 18 years, the mean serum T3 levels being within the normal adult range. By contrast, Rubenstein et al. (1973) reported higher serum T3 levels in young children with a fall of approximately 0 05 ng/ml per decade.After extensive routine use of serum T3 estimations, it has become apparent that circulating T3 levels in children are higher than in adults, and that many clinically hypothyroid children with raised serum thyroid-stimulating hormone (TSH) levels Received 18 February 1977 have T3 levels within the normal adult range. This study was undertaken to determine serum T3 and T4 levels in normal children from 3 weeks to 17 years of age. Subjects and methodsSera were obtained for estimation of T3, T4, and TSH concentrations in the following groups of children.( 1)