The concentration of thyroid hormone in preterm infants is lower than that in term infants. This phenomenon is referred to as transient hypothyroxinaemia of prematurity. Low thyroid hormone levels after very preterm birth are associated with worse developmental outcome in childhood, but only one randomized controlled trial has been carried out in the surfactant era to find out whether thyroid hormone supplementation is beneficial for developmental outcome. More studies are required to find out whether thyroid hormone supplementation is beneficial, and if so, for which preterm group.