Total and free thyroid hormone concentrations in the neonatal period

Radioimmunoassay measurements of serum concentrations of thyroxine (T4), triiodothyronine (T3), free T4 (FT4), free T3 (FT3), and thyroxine binding globulin (TBG) were conducted in full-term newborn infants between birth and 5 days of age. The mean concentrations of T4 and FT4 increased from cord blood levels of 11.9 µg/100 ml and 2.9 ng/100 ml to peak values of 16.2 µg/100 ml and 7 ng/100 ml by 24 to 48 hours of age. Mean serum total and free T3 concentrations increased from cord blood levels of 50.5 ng/100 ml and 146 pg/100 ml to peak values of 419 ng/100 ml and 1,260 pg/100 ml by 24 hours of age. Mean T3/T4 and FT3/FT4 ratios increased from 1/238 to 1/39 and from 1/20 to 1/6, respectively, during this period. By 72 to 126 hours, both the T4 and T3 concentrations had fallen somewhat. Mean serum TBG concentrations were unchanged and approximated 5.0 mg/100 ml during the first 5 days of life. These data confirm earlier reports that the normal newborn infant rapidly becomes chemically hyperthyroid in the neonatal period due to increased thyroid hormone secretion; this neonatal hyperthyroid state is due more to T3 than to T4. The data also confirm earlier reports that the fetus is T3 deficient due to a decreased capacity to monodeiodinate T4 to T3 in extrathyroidal tissues. The rapid increase in serum T3 after delivery at a time when the capacity to convert T4 to T3 is reduced suggests that the increment in serum T3 is due, predominantly, to increased T3 secretion from the thyroid gland stimulated by the neonatal TSH surge. Thus with parturition, the newborn infant is transformed from a state of chemical T3 deficiency to a state of chemical T3 thyrotoxicosis.