Serum TSH, T(4), T(3) rT(3) and resin T(3) uptake (RT(3)U) were daily or monthly measured in 71 full-term neonates and 146 healthy infants, to study the changes of these thyroid hormones levels in relation to the development during infancy. Serum TSH levels at the first day of life were very high and the geometric mean was 18.8μU/ml. Following an abrupt fall at the second day (a mean of 7.7μU/ml), the level decreased gradually to a mean of 3.4μU/ml at the third week, which was not significantly different from the values in children aged 1 to 5 years. Serum T(4) and T(7) (T(4) x RT(3)U) values remained high during the first 5 days of life (16.1-18.9μg/dl for T(4) and 5.2-6.4 for T(7)) as compared with the levels in cord blood. The levels decreased to values similar to those seen in children aged 1 to 5 years by the third week of life (10.8μg/dl for T(4) and 2.9 for T(7)). Serum T(3) levels in the first 2 days of life were high as compared with those of cord blood. Following a transient fall until the age of 4 days, the level again increased slowly and reached the maximum mean level of 208ng/dl at 3 months. The mean T(3) levels between 3 and 21 days of life (113-155ng/dl) were lower than those in children aged 1 to 5 years (169ng/dl), but the levels in infants aged 3 to 6 months (190-208ng/dl) were higher than those in children aged 1 to 5 years. On the other hand, serum rT(3) levels were remarkably high in cord blood (184ng/dl) and remained high during the first 5 days of life (158-231ng/dl). An abrupt fall occurred at the second week (a mean of 70ng/dl), and thereafter the levels decreased slowly to a mean of 26ng/dl by the age of 7 to 8 months, a value similar to that seen in children aged 1 to 5 years. The difference in the T(3)/T(4) ratio from that in children aged 1 to 5 years had become insignificant by the age of 22 to 28 days, while the differences in the rT(3)/T(4) and T(3)/rT(3) ratios had become insignificant by the age of 7 to 8 months. These results suggested that the varying maturation of the conversion mechanism of T(4) to T(3) in the peripheral tissues in developing infants together with the neonatal surge of TSH secretion may be responsible for these extraordinary changes of serum thyroid hormones during infancy, especially in the neonatal period. The present study also demonstrated that the maturation of the conversion mechanism has completed by the age of 7 to 8 months.