Research in context
Evidence before this study
In the past two decades, epidemiological studies have shown that overt maternal hypothyroidism and hypothyroxinaemia is negatively associated with adverse neurodevelopmental outcomes such as low IQ, autism, schizophrenia, and attention-deficit hyperactivity disorder. Furthermore, clinical studies suggest that maternal thyroid dysfunction might affect brain morphology, particularly grey matter. However, the most vulnerable time window during which maternal thyroid function is associated with offspring neurodevelopmental outcomes remains to be identified.
Added value of this study
In our study we identified an inverted U-shaped association of maternal thyroid-stimulating hormone with total grey matter and cortical grey matter volume as assessed by MRI scanning in the offspring, which was strongest if thyroid function was assessed in early pregnancy (<14 weeks). To the best of our knowledge, this study is the first to show that an association with a neurodevelopmental outcome is more evident if thyroid function is measured before the 14th week of pregnancy.
Implications of all the available evidence
In line with previous studies, our study shows that both exposure to a low and a high maternal thyroid function is negatively associated with child total grey matter volume and cortical grey matter volume as assessed by MRI. The absence of association from the 14th week onwards is an important novel finding that should be considered during clinical risk assessment and when timing interventions during clinical practice, as well as in the design of future studies of the effects of levothyroxine treatment for mild thyroid disease in pregnancy.