Thyroid Function in Early Pregnancy, Child IQ, and Autistic Traits: A Meta-Analysis of Individual Participant Data

Low maternal free T4 (FT4) has been associated with poor child neurodevelopment in some single-center studies. Evidence remains scarce for the potential adverse effects of high FT4 and whether associations differ in countries with different iodine status.

To assess the association of maternal thyroid function in early pregnancy with child neurodevelopment in countries with a different iodine status.

Meta-analysis of individual participant data from 9036 mother-child pairs from three prospective population-based birth cohorts: INMA [Infancia y Medio Ambiente (Environment and Childhood project) (Spain)], Generation R (Netherlands), and ALSPAC (Avon Longitudinal Study of Parents and Children, United Kingdom). The exclusion criteria were multiple pregnancies, fertility treatments, thyroid-interfering medication usage, and known thyroid disease.

Child nonverbal IQ at 5 to 8 years of age, verbal IQ at 1.5 to 8 years of age, and autistic traits within the clinical range at 5 to 8 years of age.

FT4 <2.5th percentile was associated with a 3.9-point (95% CI, -5.7 to -2.2) lower nonverbal IQ and a 2.1-point (95% CI, -4.0 to -0.1) lower verbal IQ. A suggestive association of hypothyroxinemia with a greater risk of autistic traits was observed. FT4 >97.5th percentile was associated with a 1.9-fold (95% CI, 1.0 to 3.4) greater risk of autistic traits. No independent associations were found with TSH.

Low maternal FT4 was consistently associated with a lower IQ across the cohorts. Further studies are needed to 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.