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 treate d with thyroxine in three different dosage schemes: 10, 8 and 6 mug . kg-1 birthweight . day-1 during the first 6 weeks of life. The aim was to prevent transient hypothyroxinemia of the preterm infant. Plasma l evels of thyroxine, free thyroxine, triiodothyronine, reverse triiodot hyronine. thyroxine-binding globulin and thyrotropin were measured wee kly. Thyroxine administration increased thyroxine and free thyroxine l evels most properly in the 8-mug supplementation group. It did not res ult in a change in plasma triiodothyronine levels. Levels of reverse t riiodothyronine increased in relation to the thyroxine dosage. Thyrotr opin secretion was suppressed in the 6- and 8-mug groups during the fi rst 2 weeks, while in the 10-mug group suppression lasted 4 weeks. No clinical adverse effects of thyroxine administration were seen. We con clude that 8 mug thyroxine . kg-1 birthweight . day-1 for 6 weeks prev ents transient hypothyroxinemia. The finding that plasma triiodothyron ine concentrations are not influenced by thyroxine administration sugg ests a specific maturation process in the deiodination of thyroxine.