Effects of various factors including perinatal factors, neonatal feeding method and maternal thyroid hormones levels on the pituitary-thyroid function in neonates were studied. Perinatal factors, such as cesarean section, breech delivery, twin, neonatal jaundice and neonatal asphyxia, had no effects on serum TSH levels between 10 and 28 days of life. There were also no significant differences in serum TSH or T(4) during 14-28 days of life between breast-fed and artificially fed infants. Though serum TSH levels in low birth weight neonates (preterm neonates) at the first day of life were significantly lower than those in full-term neonates, no significant difference between the two had been observed thereafter. Serum T(3) levels in preterm neonates were significantly lower and serum T(4) levels tended to be lower than those levels in full-term neonates during the first 2 weeks of life. On the other hand, in preterm neonates rT(3) levels were significantly higher and T(3)/rT(3) ratios were significantly lower than those in full-term neonates during the first 2 weeks of life. These results suggested that the conversion activity of T(4) to T(3) in the peripheral tissues is more immature in preterm neonates.
No significant correlations of serum TSH, T(4), T(3) and rT(3) levels were present between the maternal blood and cord blood, indicating that these hormones can hardly cross the placenta.