Thyroxine administration to infants of less than 30 weeks gestational age decreases plasma tri-iodothyronine concentrations

Objective: To investigate the effect on thyroid hormone metabolism of the a dministration of thyroxine to very preterm infants. Design and methods: Two hundred infants of less than 30 weeks gestation wer e enrolled into a randomized, double-blind, placebo-controlled trial. Thyro xine (T-4) (at a fixed daily dose of 8 mu g/kg birthweight) or placebo was started 12-24h after birth and discontinued 6 weeks later, Plasma concentra tions of T-4, tri-idothyronine (T-3), reverse T-3 (rT(3)) TSH, and thyroxin e-binding globulin were measured weekly during trial medication and 2 weeks thereafter. Results: The T-4 and the placebo group each comprised 100 infants. Antenata l, perinatal, and postnatal clinical characteristics were comparable in bot h groups. T-4 and rT(3) were significantly increased in the T-4 group. TSH concentrations were depressed in the T-4 group and T-3 was significantly de creased, probably as a result of TSH depression. The T-4/T-3 and T-4/rT(3) ratios differed significantly between the two study groups. Conclusions: Daily T-4 administration during the first 6 weeks after birth to infants of less than 30 weeks gestation prevents hypothyroxinemia, but d ecreases plasma T-3. concentrations. Our finding possibly implies that very preterm infants should receive supplements of both T-4 and T-3.