Thyroxine administration to infants of less than 30 weeks' gestational age does not increase plasma triiodothyronine concentrations.

Very preterm infants (less than 30 weeks' gestational age) were treated with thyroxine in three different dosage schemes: 10, 8 and 6 micrograms.kg-1 birthweight.day-1 during the first 6 weeks of life. The aim was to prevent transient hypothyroxinemia of the preterm infant. Plasma levels of thyroxine, free thyroxine, triiodothyronine, reverse triiodothyronine, thyroxine-binding globulin and thyrotropin were measured weekly. Thyroxine administration increased thyroxine and free thyroxine levels most properly in the 8-micrograms supplementation group. It did not result in a change in plasma triiodothyronine levels. Levels of reverse triiodothyronine increased in relation to the thyroxine dosage. Thyrotropin secretion was suppressed in the 6- and 8-micrograms groups during the first 2 weeks, while in the 10-micrograms group suppression lasted 4 weeks. No clinical adverse effects of thyroxine administration were seen. We conclude that 8 micrograms thyroxine.kg-1 birthweight.day-1 for 6 weeks prevents transient hypothyroxinemia. The finding that plasma triiodothyronine concentrations are not influenced by thyroxine administration suggests a specific maturation process in the deiodination of thyroxine.