Serum free thyroxine and free tri-iodothyronine in normal children

Serum free thyroxine (fT4) and free tri-iodothyronine (fT3) were measured in 138 normal school children, adolescents and their younger siblings, whose ages ranged from 3 months to 18 years. Mean fT4 concentration (16.8 pmol/L) was similar to the adult mean concentration of 17 pmol/L and all the values were within the adult reference range. At all ages the fT3 range was considerably higher than the adult reference range and the overall mean fT3 concentration (8.3 pmol/L) was at the upper limit of normal in adults. In subjects aged 13-18 years the mean fT4 concentration was higher and the mean fT3 concentration lower than in children aged 0-12 years. The reasons for these differences are not known, but the data obtained provide a useful guide to the interpretation of serum free thyroid hormone measurements in children.