[Importance of the excretion of triiodothyronine (T3) in normal infants and children].

Serum concentrations of thyroxine (T4) and triiodothyronine (T3) are higher in normal neonates and children than in adults, having a thyroxine to triiodothyronine ratio close to 50. T3 excretion is almost as high as that of T4 (T4 to T3 ratio from 0.6 to 0.8 in children). This amount of excreted T3 represents a loss of about 1/10 of the theoretical daily production rate of the hormone.