A new review covers recent findings on chemicals that disrupt thyroid hormone function and their effects on brain development[i]. The review describes how numerous common chemicals can interfere with normal thyroid hormone actions in pregnant women, which are essential for normal brain development in foetuses and young children.
Thyroid hormone is essential for normal brain development, where it influences a number of processes that occur during early pregnancy, after birth, and throughout life. During the first 10 to 12 weeks of pregnancy, a foetus depends entirely on its mother for thyroid hormone. Hence, severe maternal thyroid hormone deficiency may affect the neurodevelopment of the foetus. In fact, recent epidemiological studies have shown that only moderate disruption of thyroid hormones in the mother is sufficient to increase the risk of neurodevelopmental disorders in the child. Endocrine disrupting chemicals (EDCs) can disrupt thyroid function, and the review aimed to discuss how different EDCs can affect brain development.
Chemicals discussed in the review include perchlorate, phenols, pesticides, polychlorinated-biphenyls (PCBs), poly brominated flame-retardants, perfluorinated compounds (PFCs), and phthalates. Many substances falling into these categories have been banned, but remain environmentally relevant, due to the high volumes that were produced and their long persistence in the environment.
For each chemical class considered, the reviewers discuss the thyroid hormone disrupting effects and the main mechanisms by which they could modify hormone signaling and lead to developmental effects.
The reviewers conclude that their report adds to the established knowledge on the importance of iodine and thyroid hormone in brain development in offspring. They propose that prenatal exposure to mixtures of thyroid hormone-disrupting chemicals provides a plausible biological mechanism contributing to current increases in incidence of neurodevelopmental conditions, including autism spectrum disorder and attention deficit hyperactivity disorders.
[i] Mughal, B. B., Fini, J.-B. & Demeneix, B. Thyroid disrupting chemicals and brain development: an update. Endocr Connect (2018). doi:10.1530/EC-18-0029 http://www.endocrineconnections.com/content/early/2018/03/19/EC-18-0029.long (Accessed 5 April 2018)