Context: Transient hypothyroxinemia of prematurity (THoP) has been associated with neurodevelopmental impairment in infancy and childhood. It is not known whether these relations persist into adulthood. Objective: The objective was to examine whether there is an effect of THoP on intelligence quotient (IQ) score and motor functioning at a young adult age. Design: This study was part of the 19-year follow-up of the Project On Preterm and Small-forgestational-age birth (POPS) cohort, which included infants born very preterm (ie,32 wk) and/or with a very low birth weight (ie, 1500 g). Setting: This was a multicenter study. Patients: There were 398 19-year-old participants of the POPS cohort, of whom 120 had THoP. Exposure: T4 concentrations were obtained through the national neonatal screening program for congenital hypothyroidism.THoP was defined as a total T4 concentration