Abstract
Low maternal free thyroxine (FT4) has been associated with poor child neurodevelopment in some single-centre studies. Evidence remains scarce for potential adverse effects of high FT4 and whether associations differ in countries with a different iodine status. To assess the association of maternal thyroid function in early pregnancy with child neurodevelopment in countries with a different iodine status. Design, Setting and Participants:Meta-analysis of individual-participant data compromising 9,036 mother-child pairs from three prospective population-based birth cohorts: INMA (Spain), Generation R (The Netherlands) and ALSPAC (United Kingdom). Exclusion criteria were multiple pregnancies, fertility treatments, thyroid interfering medication usage, and known thyroid disease. Main outcomes:Child non-verbal IQ at 5-8 years of age, verbal IQ at 1.5-8 years of age, and autistic traits within the clinical range at 5-8 years of age. Results: FT4 <2.5th percentile was associated with a 3.9 [95% confidence interval -5.7 to -2.2)] point lower non-verbal IQ and a 2.1 (-4.0 to -0.1) point lower verbal IQ. A suggestive association of hypothyroxinemia with a higher risk of autistic traits was observed. FT4 >97.5th percentile was associated with a 1.9 (1.0 to 3.4) fold higher risk of autistic traits. No independent associations were found with thyrotropin.Low maternal FT4 was consistently associated with lower IQ across cohorts. Further studies should replicate the findings of autistic traits and investigate the potential modifying role of maternal iodine status. FT4 seems a reliable marker of fetal thyroid state in early pregnancy, regardless of the type of immunoassay