Effect of low thyroid hormone bioavailability on childhood cognitive development: data from the Avon Longitudinal Study of Parents and Children birth cohort

Abstract Background Hypothyroidism is associated with impaired cognitive development. Whether free thyroxine (T4 ) concentrations at the lower end of the reference range also have a negative effect is less clear. A common genetic variant, the Thr92Ala substitution in deiodinase-2 can exacerbate low free T4 concentrations by reducing intracellular conversion of free T4 to free tri-iodothyronine. We aimed to see whether low thyroid hormone bioavailability (free T4 in the lowest quartile and the Thr92Ala substitution in deiodinase-2) in childhood adversely affects cognitive development. Methods The data from this study come from a retrospective cross-sectional analysis in a population-based birth cohort study, the Avon Longitudinal Study of Parents and Children. Children had thyroid function tested at age 7 years and extensive cognitive assessments at age 8 years. We assessed the odds of having a lower end intelligence quotient (IQ) (