Introduction: Maternal thyroid function in early pregnancy is crucial for fetal neural development. Early offspring language acquisition is an important indicator of fetal neural development and a predictor of future cognitive development. We aimed to investigate how the mothers thyroid function and the presence of thyroid autoimmunity in early pregnancy are associated with offsprings language acquisition at the age of 12-37 months.
Methods: This study was embedded in the prospective Odense Child Cohort (OCC). Mothers were excluded in case of known thyroid disease. A blood sample was drawn at median 12th gestational week (range: 8-20) and analyzed for free thyroxine (FT4), thyrotropin, and anti-peroxidase antibodies (TPOAb). The Danish adaption of the MacArthur-Bates Communicative Development Inventories (MB-CDI) parent reports was assessed every third month when the child was 12-37 months and was used to evaluate the offsprings vocabulary. Data from all completed MB-CDI reports (productive vocabulary) for each child was included in the analysis. Associations between maternal FT4, thyrotropin, and TPOAb, respectively, and scores in the MB-CDI were evaluated by conditional growth models, with thyroid variables as predictors, estimating inter-individual variability in intra-individual patterns of change (in MB-CDI-scores) over time (offspring age) and main effects of covariates.
Results: The study included 735 mother-child pairs (383 boys and 352 girls). The probability of producing words within the MB-CDI at the age of 12 to 37 months correlated negatively with maternal FT4 in both girls and boys (P < 0.001). Likewise, language acquisition was better in both girls and boys when maternal FT4 was below the 20th percentile compared to children exposed to a maternal FT4 above the 80th percentile (P < 0.001, P = 0.002, respectively). Language acquisition was better for girls with maternal thyrotropin >2.5 mIU/L than for those with maternal thyrotropin ≤2.5 mIU/L (P < 0.001). The same trend was evident for boys after the age of 27 months (P < 0.001). Language acquisition at age 12-37 months for both girls and boys was inversely correlated with TPOAb levels, as TPOAb >11 kIU/L decreased the probability of producing words compared to those with maternal TPOAb levels <11 kIU/L (P < 0.001).
Conclusion: Language acquisition in offspring at age 12-37 months correlated inversely with maternal FT4 and serological markers of thyroid autoimmunity in early pregnancy. Thus, thyroid autoimmunity per se may have a negative impact on fetal neural development. However, the finding that maternal thyroid function in the lower-normal range may be favorable for language acquisition contrasts with the current perception.