Neurodevelopmental Outcomes at 42 Months After Thyroxine Supplementation in Infants Below 28 Weeks' Gestation: A Randomized Controlled Trial

Infants below 28 weeks' gestation have low thyroid hormone plasma levels compared with more mature infants and this may contribute to their risk of developmental disability. We aimed at determining the effect of supplementation with levothyroxine (LT4) for extremely premature infants born below 28 weeks' gestations on neurodevelopmental outcomes at 42 months. An explanatory double-blind, randomized, placebo-controlled trial consecutively recruited 153 infants below 28 weeks' gestation from 5 neonatal units in the United Kingdom. Infants were either supplemented with LT4 started intravenously during the first 5 days after birth and then changed to oral LT4 when enteral feeds were fully established (8 μg/kg birthweight/day as a single daily dose) or given placebo until 32 weeks' corrected gestational age. Neurodevelopmental outcomes at 42 months (range 40-43) were evaluated in 59 of these infants (30 LT4-supplemented, 29 placebo) by using Bayley III Mental and Psychomotor Developmental Indices. Cognition outcomes was correlated with plasma free thyroxine (fT4) level at 36 weeks and diffusion tensor imaging (DTI) markers. The LT4 supplemented group performed significantly better in motor, language, and cognitive function domains. The mean of the difference between each group (95% confidence intervals [CI], -value) was motor domain 6.96 ([0.55-13.38],  = 0.034); language domain 8.93 ([0.16-17.70],  = 0.041); and cognition domain 6.35 ([0.14-12.55],  = 0.045). Neurodevelopmental outcome at 42 months had some associations with the trial's primary outcome (subarachnoid space width and motor outcome,  = 0.03), plasma fT4 level at 36 weeks (fT4 and cognition outcome,  = 0.01), and DTI at 36 weeks with cognition outcomes ( > 0.05). Our data suggest that early supplementation with LT4 may improve long-term neurodevelopment in infants born below 28 weeks' gestation, but larger trials are warranted as the current reported improvements shown are not strong enough to warrant a change in practice.