Abstract
Tg, TSH, free T4 index, and free T3 index were determined in cord serum and related to birth weight and sex in infants with weights of less than 2500 grams, 19 males and 19 females. Free index values are the product of the T3 resin uptake ratio and the appropriate thyroid function indice. Mean ± SD birth weight of male (1850 ± 413 grams) and female (2032 ± 301 grams) infants did not differ significantly (p >0.05). Similarly, mean ± SD cord serum Tg (124 ± 49 vs 107 ± 55 ng/ml), TSH (9.2 ± 6.5 vs 7.1 ± 2.7 μU/ml), free T4 index (6.2 ± 1.6 vs 6.1 ± 1.0 μg/dl), and free T3 index (47 ± 12 vs 47 ± 5 ng/dl) values of male and female infants did not differ significantly (p >0.1). Free T3 index values of male (r = 0.570, p <0.01) and female (r = 0.443, p <0.05) infants correlated positively with the free T4 index values. Tg levels and the log of TSH levels correlated negatively with birth weight in male (Tg, r = -0.849; log TSH, r = -0.660) and female (Tg, r = -0.891; log TSH, r = -0.600) infants (p <0.005). Also, Tg levels correlated positively with the log of TSH levels in male (r = 0.412, p <0.05) and female (r = 0.554, p <0.01) infants. These data are consistent with a hypothesis that changes in body composition accompanying increasing birth weight are associated with an increase in thyroid gland responsiveness to TSH or a decrease in T4 clearance.
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