Very preterm infants (less than 30 weeks' gestational age) were treated with thyroxine in three different dosage schemes: 10, 8 and 6 μg·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-μg 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-μg groups during the first 2 weeks, while in the 10-μg group suppression lasted 4 weeks. No clinical adverse effects of thyroxine administration were seen. We conclude that 8 μg 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.