Everyday chemicals linked to development disorders in children

Exposure of pregnant women and children to common thyroid-hormone-disrupting toxins may be linked to the increased incidence of brain development disorders, according to a review by French researchers.

Maternal thyroid hormones (TH) are essential for normal brain development of children, with studies indicating that even moderate disruption to TH function in pregnant women may affect cognitive development and increase the risk of brain developmental disorders in their children. In modern times, an increase in chemical production has led to widespread environmental chemical contamination that can affect normal hormone function in those exposed — particularly in vulnerable populations such as children and pregnant women.

“The chemicals that interfere with thyroid hormone signalling can do so because they are very similar in structure to thyroid hormones, so they can ‘trick’ the body into thinking they are thyroid hormones, and consequently disrupt the normal thyroid hormone signalling that directs brain development,” explained Associate Professor Samantha Richardson from RMIT University, who was not involved in the study. “Because these chemicals are not actually thyroid hormones, they can block the real thyroid hormones from doing their job, ie, normal brain development.”

Many endocrine-disrupting chemicals (EDCs), which include pesticides and substances used in manufacturing a multitude of products, have been reported to interfere with thyroid hormone function. Now, Professor Barbara Demeneix and colleagues at the Université Paris-Sorbonne have examined published evidence of the wide variety and high number of EDCs — from pesticides to chemicals used in the manufacture of drugs, cosmetics, furniture and plastics — that can all interfere with TH.

Published in the journal Endocrine Connections, the team’s findings indicate that exposures of pregnant women and children to thyroid-disrupting chemicals in the environment pose risks for child development and health, and underline the need for a more targeted public health intervention strategy.

“We have reviewed the documented exposures of pregnant women and children to mixtures of thyroid-hormone-disrupting chemicals and propose that the datasets provide a plausible link to the recent increased incidence of neurodevelopmental conditions, including autism spectrum disorders and attention deficit hyperactivity disorders,” Professor Demeneix said.

“Many experts in the field consider that the current testing guidelines for thyroid-disrupting chemicals are not sufficiently sensitive, do not take into account recent findings and do not adequately consider risks to vulnerable populations, such as pregnant women.”

One such expert, Professor Ian Rae from the University of Melbourne, explained, “When these substances were invented and registered for use they passed the ‘acute toxicity’ test. However, increasingly they are suspected of causing chronic affects for which there is no obvious infective agent, like a bacterium or a virus.”

Professor Rae acknowledged that cause and effect is “almost impossible to establish, because we can’t deliberately dose people to see what happens”, but did note that pressure is growing to phase out substances suspected of chronic effects and replace them with safer chemicals.

“It’s a slow road,” he said, “along which manufacturers and regulators are harried by scientists producing results like this.”

So what kinds of chemicals are we talking about? According to Associate Professor Richardson, “Examples of common thyroid hormone-disrupting chemicals include flame retardants in carpets, curtains, furniture and non-flammable clothing; plastic containers for heating food; and ‘non-stick’ agents. Even though some of these chemicals are no longer made, they are still in the environment because the items containing them are still in use.”

Professor Stuart Khan from UNSW meanwhile claimed that Australian water quality managers should pay attention to studies such as these, noting that “practically all of the contaminants discussed in this paper are known drinking water and wastewater contaminants”. These include perchlorate, Bisphenol A and other phenols, brominated flame retardants, triclosan, the various pesticides, phthalates, perfluorinated chemicals and polychlorinated biphenyls (PCBs).

“Many of these chemicals … are known to accumulate in the sludge produced by sewage treatment plants,” Professor Khan said. “Due to their persistence in the environment, we need to be very careful about how we dispose of, or re-use, that sludge. In many cases, it is re-used for agriculture, so understanding the risks associated with potential accumulation in food crops is an essential aspect of managing the risks of exposure to these chemicals by people.”

But Dr Ian Musgrave from the University of Adelaide was more sceptical about the study, particularly given the researchers’ reference to “recent increased incidence of neurodevelopmental conditions”. He noted that “it is widely accepted that the ‘increase’ in autism and to some extent ADHD is due to the combination of better diagnosis, diagnostic substitution ... and with ADHA, some degree of overdiagnosis” — there is, therefore, “nothing to explain”.

Professor Creswell Eastman AM, from Sydney Medical School, meanwhile noted that the researchers used experimental animal data in most instances and then extrapolated to humans — “so some of their material may be relevant but not all of it”. Robert Norman from the University of Adelaide added that the link between thyroid function and exposure to hormone-disrupting chemicals remains tenuous, though he believes the hypothesis deserves more study in humans.

“Regardless, we do need to reduce the exposure of pregnant women to any of these chemicals and early testing of thyroid function in all women thinking of getting pregnant is warranted,” Norman concluded.

The news comes shortly after separate revelations that vitamin D plays an important role in the brain development of children, with low levels of vitamin D during pregnancy and breastfeeding potentially related to autism-like social behaviour in later life.