Infants born to diabetic mothers have decreased activity of many metabolic pathways which might be regulated by thyroid hormone. Serum TSH, T4, T3, and reverse T3 levels were measured in 22 term infants of diabetic mothers and in 9 normal term babies at 2, 12, 24, and 72 hours of age, as well as in maternal and cord sera. T4 binding index and free T4 levels were measured in 11 diabetic mothers and their babies and 5 normal mothers and babies. Mean TSH levels did not differ between diabetic and normal mothers or their infants. Mean T4 of the diabetic mothers (9.6 micrograms/dl) was significantly (p less than 0.005) less than the mean T4 of the normal mothers (12.8 micrograms/dl). Mean T4 of neonatal specimens was lower in infants of diabetic mothers for each determination, but this difference achieved statistical significance at the 12-hour sample only (p less than 0.001). Mean serum T4 binding index was similar in the neonatal specimens at each time period studied. Mean T3 of diabetic mothers (149 ng/dl) was significantly (p less than 0.001) less than that of normal mothers (217 ng/dl). At each time interval, mean T3 concentration in infants of diabetic mothers was significantly lower than that of normal infants. Levels of reverse T3 were not significantly different between normal and diabetic mothers or their neonates. These data suggest that there is an effect of maternal diabetes on T3 secretion or conversion of T4 to the more active hormone, T3, in the fetus and early newborn.(ABSTRACT TRUNCATED AT 250 WORDS)