Serum thyroxine and triiodothyronine levels in newborns: effect of gestational age

Serum T₄ and T₃ were measured in 31 newborns of gestational ages 32–43 weeks. Out of 31, 11 were term, 12 preterm and 8 postterm. Blood samples were taken from cord blood and from peripheral vein at 24 and 72 hours of age. Serum T₄ and T₃ values were low in cord blood samples, raised in 24 hours samples and then declined in 72 hour samples. In pretern newborns cord T₄ and T₃ values were significantly lower and there was a blunted rise and fall in 24 and 72 hours samples as compared to term newborns. In post term newborns cord serum T₄ and T₃ values were significantly raised while in peripheral vein samples difference was statistically insignificant as compared to term newborns. This high incidence of low cord values and transient hypothyroxinemia observed in preterm and postnatal surge of T₄ may give rise to false results while screening for hypothyroidism.