Importance: Maternal thyroid hormone insufficiency during pregnancy can affect children's cognitive development. Nevertheless, the behavioral outcomes of children exposed prenatally to mild thyroid hormone insufficiency are understudied.
Objective: To examine whether exposure to maternal mild thyroid hormone insufficiency in early pregnancy was related to symptoms of attention-deficit/hyperactivity disorder (ADHD) in children at 8 years of age.
Design, setting, and participants: The study was embedded within the Generation R, a population-based birth cohort in the Netherlands. Children in the Generation R Study are followed up from birth (April 1, 2002, through January 31, 2006) until young adulthood. Of the 4997 eligible mother-child pairs with data on maternal thyroid levels (excluding twins), 3873 pairs of children and caregivers (77.5%) visited the Generation R research center for in-depth assessments and were included in the main analyses. Data collection in Generation R started December 1, 2001 (enrollment of pregnant women), and is ongoing. For this study, we used the data that were collected until January 1, 2014. Data analyses started on January 31 and finished June 30, 2014.
Main outcomes and measures: Maternal hypothyroxinemia, characterized by low levels of free thyroxine coexisting with reference thyrotropin levels, and children's symptoms of ADHD. Maternal thyroid hormone levels (thyrotropin, free thyroxine, thyroid peroxidase antibodies) were measured at a mean (SD) of 13.6 (1.9) weeks of gestation. Children's ADHD symptoms were assessed at 8 years of age using the Conners' Parent Rating Scale-Revised Short Form; higher scores indicate more ADHD symptoms (possible range, 0-36).
Results: Maternal hypothyroxinemia (n = 127) in early pregnancy was associated with higher scores for ADHD symptoms in children at 8 years of age after adjustments for child and maternal factors (ie, sex, ethnicity, maternal age, maternal educational level, and income) (increase in ADHD scores, 7% [95% CI, 0.3%-15%]). The results remained essentially unchanged when women with elevated levels of thyroid peroxidase antibodies were excluded from the analyses (increase in ADHD scores, 8% [95% CI, 1%-16%]). Additional adjustment for children's IQ or comorbid autistic symptoms attenuated the association (increase in ADHD scores adjusted for autistic symptoms, 7% [95% CI, 1%-15%]; increase in ADHD scores adjusted for IQ, 6% [95% CI, 1%-14%]).
Conclusions and relevance: Children exposed to maternal hypothyroxinemia in early pregnancy had more ADHD symptoms, independent of confounders. This finding suggests that intrauterine exposure to insufficient thyroid hormone levels influences neurodevelopment in offspring.