Context: Transient hypothyroxinaemia of prematurity (THoP) has been associated with neurodevelopmental impairment in infancy and childhood. It is not known whether these relations persist into adulthood. Objective: To examine whether there is an effect of THoP on intelligence quotient (IQ) score and motor functioning at young adult age. Design: This study was part of the 19-year follow-up of the Project On Preterm and Small-for-gestational-age birth (POPS) cohort, which included infants born very preterm (i.e.