Comparison of T4, T3, rT3 and TSH concentrations in cord blood and serum of infants up to 3 months of age.

T4, T3, TSH and rT3 concentrations were measured by radioimmunoassay in cord and postnatal (8--94 days of age) serum samples from randomly selected normal newborn infants (Group I). T4 and TSH levels also were determined in cord and postnatal sera from an additional group of apparently healthy infants 8--260 days of age, whose cord serum T4 levels were in the upper or lower 10% of the normal range of values (Group II). Postnatal T4, T3 and TSH concentrations were stable over this age range; there were no significant differences between male and female infant samples. However, there was a significant decrease in serum rT3 concentrations from 8 to 50 days of age. For the Group I infants, there were significant positive correlations between cord serum T4 and postnatal serum T4 levels, cord serum TSH and postnatal serum TSH levels, and cord serum rT3 and postnatal serum rT3 concentrations. For Group II infants, a significant positive correlation was found for cord T4--postnatal T4 serum concentrations.