Cord serum thyroid-stimulating hormone and thyroglobulin levels decline with increasing birth weight in newborns.

Cord serum thyroglobulin (Tg) and TSH levels were related to birth weight in 3 groups of newborn infants composed of 101 infants. Serum free T3 index, free T4 index, and/or T4 also were determined. Group I consisted of normal term newborns (20 females and 19 males), whose mean +/- SD gestational ages (40.1 +/- 0.7 vs. 40.1 +/- 0.5 weeks) did not differ, but whose mean birth weights (3299 +/- 282 vs. 3757 +/- 447 g) differed significantly (P less than 0.005). In female infants, serum Tg levels (r = -0.401; P less than 0.05) and the log of TSH levels (r = -0.576; P less than 0.005) correlated negatively with birth weight, while Tg levels correlated positively with the log of TSH levels (r = 0.401; P less than 0.05). In contrast, none of these correlations was significant for male infants. However, T4 levels and birth weight correlated positively (r = 0.499; P less than 0.025) in male infants, but not in female infants. Group II consisted of newborns whose birth weights were less than 2500 g (19 females and 19 males). Mean birth weights of female (2032 +/- 301 g) and male (1850 +/- 413 g) infants did not differ significantly (P greater than 0.05). Both the Tg levels and the log of the TSH levels correlated negatively with birth weight in female (Tg, r = -0.891 and P less than 0.005; log TSH, r = 0.600 and P less than 0.005) and male (Tg, r = -0.849 and P less than 0.005; log TSH, r = -0.660; P less than 0.005) infants. Also, Tg levels correlated positively with the log of the TSH levels in female (r = 0.554; P less than 0.01) and male (r = 0.412; P less than 0.05) infants. Free T4 index levels correlated positively with free T3 index levels in female (r = 0.443; P less than 0.05) and male (r = 0.570; P less than 0.01) infants. Group III consisted of 12 normal female term newborns whose mean birth weight (3685 +/- 623 g) was not significantly (P less than 0.2) different from that of the males of group I, and 12 normal male term newborns whose mean birth weight (4104 +/- 248 g) was significantly (P less than 0.005) greater than that of the males of group I. Unlike in lower weight female or male infants, serum Tg levels did not correlate with birth weight or the log of TSH levels.(ABSTRACT TRUNCATED AT 400 WORDS)