新生児,乳児期における甲状腺機能に関する研究

Abstract

Serum TSH, T4, T3 rT3 and resin T3 uptake (RT3U) 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 18.8 μU/ml. Following an abrupt fall at the second day (a mean of 7.7 μU/ml), the level decreased gradually to a mean of 3.4 μU/ml at the third week, which was not significantly different from the values in children aged 1 to 5 years.Serum T4 and T7 (T4 x RT3U) values remained high during the first 5 days of life (16.1-18.9 μg/dl for T4 and 5.2-6.4 for T7) 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 (10.8 μg/dl for T4 and 2.9 for T7). Serum T3 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 208 ng/dl at 3 months. The mean T3 levels between 3 and 21 days of life (113-155 ng/dl) were lower than those in children aged 1 to 5 years (169 ng/dl), but the levels in infants aged 3 to 6 months (190-208 ng/dl) were higher than those in children aged 1 to 5 years. On the other hand, serum rT3 levels were remarkably high in cord blood (184 ng/dl) and remained high during the first 5 days of life (158-231 ng/dl). An abrupt fall occurred at the second week (a mean of 70 ng/dl), and thereafter the levels decreased slowly to a mean of 26 ng/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 T3/T4 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 rT3/T4 and T3/rT3 ratios had become insignificant by the age of 7 to 8 months.These results suggested that the varying maturation of the conversion mechanism of T4 to T3 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.