Correlation between gestational age, weight, or ponderosity and serum thyroxine concentration at birth.

Thyroxine levels were measured in cord blood sera from 2,1800 healthy infants born of "low-risk" mothers in Los Angeles. Variations of T4 with gestational age, birth weight, ponderosity (size related to weight), sex, and race were studied, as well as the variation of W and P with GA. Over the range of GA 30 to 47 weeks, serum T4 concentrations, W, and P each increase with GA, leveling off to approximately constant values from 42 to 47 weeks; the increases are 12, 66, and 20%, respectively. For a fixed GA, T4 increases with increasing W but is not correlated with P; W, GA, and their product are useful measurements for predicting T4, but P is not. Formulas for predicting cord T4 are given for possible use in improving computer-assisted screening programs for congenital hypothyroidism; mean T4 values (+/- I SD) are tabulated against W and GA.