Background: Autism spectrum disorder (ASD) is a neurodevelopmental condition with unknown etiology. Both genetic and environmental factors have been associated with ASD. Environmental exposures during the prenatal period may play an important role in ASD development. This narrative review critically examines the evidence for a relationship between maternal thyroid dysfunction during pregnancy and ASD in the child. Summary: Studies that assessed the associations of hypothyroidism, hyperthyroidism, hypothyroxinemia, thyroid hormone concentrations, or autoimmune thyroid disease with ASD outcomes were included. Most research focused on the relationship between hypothyroidism and ASD. Multiple population-based studies found that maternal hypothyroidism was associated with higher likelihood of an ASD diagnosis in offspring. Associations with other forms of maternal thyroid dysfunction were less consistent. Findings may have been affected by misclassification bias, survival bias, or publication bias. Studies using medical records may have misclassified subclinical thyroid dysfunction as euthyroidism. Two studies that assessed children at early ages may have misclassified those with ASD as typically developing. Most studies adjusted for maternal body mass index (BMI) and/or mental illness, but not interpregnancy interval or pesticide exposure, all factors associated with fetal survival and ASD. Most studies reported a combination of null and statistically significant findings, although publication bias is still possible. Conclusions: Overall, evidence supported a positive association between maternal thyroid dysfunction during pregnancy and ASD outcomes in the child, especially for hypothyroidism. Future studies could reduce misclassification bias by using laboratory measures instead of medical records to ascertain thyroid dysfunction and evaluating children for ASD at an age when it can be reliably detected. Survival bias could be further mitigated by adjusting models for more factors associated with fetal survival and ASD. Additional research is needed to comprehensively understand the roles of maternal levothyroxine treatment, iodine deficiency, or exposure to thyroid-disrupting compounds in the relationship between maternal thyroid dysfunction and child ASD outcomes.
Keywords: autism spectrum disorder; delayed effects; hyperthyroidism; hypothyroidism; prenatal exposure; thyroid dysfunction.