It is well established that during pregnancy maternal hypothyroidism may have negative effects on the developing fetal brain. New data presented at the 2015 International Thyroid Congress and Annual Meeting of the American Thyroid Association (ITC/ATA) suggests that maternal hyperthyroidism, or higher than normal levels of thyroxine (T4), in early pregnancy may also have negative effects, including lower IQ, in offspring assessed at six years of age.
In addition, the researchers analyzed MRI data obtained from more than 600 children. Â They observed a reduction in total gray matter and cortical volume when maternal free thyroxine concentration was less than the tenth percentile or more than the 90th percentile.
We typically assess thyroid function by measuring levels of TSH or thyroid stimulating hormone; however, in this study, levels of free T4 — but not TSH — correlated with the child’s IQ.  Thus, these findings underscore the need for monitoring free T4 levels during pregnancy, in addition to the more commonly monitored TSH.
Ruta Nonacs, MD PhD
High Maternal Thyroid Function Affects Offspring Brain Development (Medscape)
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