Free triiodothyronine measured in dried blood spots from normal, low-birth-weight, and hypothyroid neonates.

Free triiodothyronine (FT3) has rarely been studied in neonates, primarily because of obvious problems in obtaining serum samples from neonates for research purposes. We adapted an analog radioimmunoassay for measuring FT3 in dried blood samples and used it to assay 361 samples selected from those collected for the screening for neonatal hypothyroidism. The mean FT3 concentration in healthy neonates on the fourth postpartum day is 2.55 pmol per liter of whole blood (SD 0.78 pmol/L, n = 145), the same as in healthy adults. Low-birth-weight infants have a decreased concentration of FT3, and this decrease is much greater in premature neonates than in infants at low weight for gestational age. In six hypothyroid newborns, the FT3 concentration was normal or increased, clearly indicating that FT3 assay is not a diagnostic tool for hypothyroidism. FT3 was barely detectable in one case of congenital galactosemia.