Nowadays, there is concern about endocrine-disrupting chemicals, especially their interference on the thyroid gland. The impact on thyroid hormone levels, especially for pregnant women during the first three months of pregnancy, may result in neurodevelopmental diseases, autism and IQ loss in the unborn child. We spoke to Barbara Demeneix, Professor from the French National Museum of Natural History, to ask why these chemicals affect the signalling of thyroid hormones and what we can do to protect our children.
The environment affects our genes
Exposure to endocrine disruptors occurs on a daily basis. We should all pay attention to it, but especially pregnant women. This is because the levels of thyroid hormone that mothers have is crucial for brain development in the unborn child. Without the right amount of the hormone at the right time, the child has an increased chance of autism and reduced IQ. The problem is that many chemicals to which we are all exposed can interfere with thyroid hormone signalling.
Autism is a developmental and neurological disease that is present from early childhood and is characterised by difficulties in communicating, forming social relationships with other people, and using language and abstract concepts.
While cretinism (which in severe cases was characterised by an IQ of around 35) has been virtually eradicated worldwide due to therapies that correct thyroid dysfunction (such as postnatal T4 therapy), there is concern that IQ loss and autism risk are increased by exposure to thyroid disrupting chemicals and iodine deficiency, especially during pregnancy.
Studies in the United States have shown a steep increase in the number of children diagnosed with autism since the early 2000s. The same dataset also shows a plateau from 2010 to 2012. Many of the children diagnosed with autism also have intellectual deficiency.
So, why have the numbers risen so steeply and what happened to make them plateau? A number of arguments have been put forward to explain the increase, including better diagnosis and increased awareness.
“But changes in diagnostic methods did not occur between 1994 and 2010,” Professor Demeneix points out. In fact, major changes did not occur until the fifth edition of the American Psychiatric Association’s (APA) Diagnostic and Statistical Manual of Mental Disorders (DSM-5) was adopted in 2013. DSM-5 introduced the concept of autism spectrum disorder, which encompasses autistic disorder (autism), Asperger’s disorder, childhood disintegrative disorder and pervasive developmental disorder from DSM-IV.
”At the beginning of the 2000s, there was a general increase in awareness of autism, as it was being extensively discussed in the media. So, the contribution of increased awareness could have first increased and then stagnated during the plateau period from 2010 to 2012,” she tells.
A more plausible reason for the plateau could be that there is now better regulation of certain chemicals. “It is not illogical that the environment has an impact on us and on how our genes are transcribed. For example, if we look at the levels of lead in the environment, it is clear that they have decreased in large part due to more comprehensive regulation. For other compounds, it is slightly more difficult to assess, so it would be worth comparing levels of certain perfluorinated compounds and manmade polychlorinated biphenyls (PCBs) before and after they were phased out,” she adds.
A biologically plausible mechanism
Anything interfering with the levels of maternal thyroid hormone will have a long-term impact on the formation and development of organs and neurons in the foetus.
Professor Demeneix suggests that Austin Bradford Hill’s nine criteria to establish epidemiologic evidence of a causational relationship are pertinent. Many of the criteria can be aptly applied to show that endocrine disruptors can be linked to biological mechanisms that could cause autism and IQ loss.
“First, there are consistent findings on endocrine-disrupting effects across different populations and places as well as coherence between epidemiology and laboratory findings through experimental evidence,” Professor Demeneix argues.
“Second, the temporality criterion requires exposure before diagnosis, as is the case of neurodevelopmental diseases generated after exposure to endocrine disruptors,” she tells.
“Third, the biological gradient suggests that greater exposure usually leads to greater effects; however, this is not always the case with endocrine disruption, as effects can be seen at low doses and the presence of mixtures of chemicals makes assessing individual effects difficult. Last but not least, there is a plausible mechanism between endocrine disruptors and their interference with thyroid hormone signalling. “Combining the facts that first, maternal hypothyroidism increases the risks of autism and second, that exposure to many chemicals reduces circulating thyroid hormones provides a strong case for biological plausibility,” Professor Demeneix explains.
Maternal thyroid function affects development
One of the major findings on thyroid hormones in the last two decades is the significance of the need for strict control of maternal levels in the first three months of pregnancy. Recent data has proven that these maternal levels of thyroid hormone in the first 10 to 12 weeks of pregnancy are determinant for the child’s brain development.
During this period, the foetal thyroid gland isn’t fully functional yet, so the foetus depends on the mother’s supply of thyroid hormones. “If there is any maternal lack of thyroid hormones, the foetus will not be able to compensate for it. This is the time when the baby is being formed, which is why women are advised not to take any medication in these first three months,” says Professor Demeneix.
Consequently, low levels of maternal thyroid hormone will negatively affect the child’s neurodevelopment. “In fact, both high or low maternal levels of thyroid hormone (even within normal values) increase the risk of having a child with an IQ less than 85. Clinical hypothyroidism during pregnancy increases the risk of autism, too. As thyroid problems are difficult to treat during pregnancy, it is important for women to get their iodine levels and thyroid status checked before becoming pregnant”.
Boys more likely to suffer
The data collected in 2010 and 2012 shows that 1 in 68 children were diagnosed with autism in the USA. For boys, this figure was as high as 1 in 48. For attention deficit/hyperactivity disorder (ADHD), current figures suggest that 14 % of American boys are affected. So, why are boys more likely to suffer from neurodevelopmental problems?
According to Professor Demeneix, there are several theories for this. “It could be that boys are more affected because they lack the second X chromosome, which is linked to many related diseases affecting the brain. They could also be more affected by an overproduction of testosterone either by the foetus or the mother.”
“A third idea is that, as testosterone aromatises to oestrogen in the brain, it has a different effect on different areas of the brain. Aromatase is an enzyme that is very sensitive to endocrine disruption,” she continues.
“A fourth, less likely factor, could be that male and female foetuses take up endocrine disruptors differently.”
There could also be a factor related to the societal expectations of girls versus boys. “Indeed, we often expect more controlled and less disturbing behaviour from girls than we do from boys, but these sociological factors are more difficult to assess,” Professor Demeneix states.
What actions could be taken?
With so many chemicals still on the market there is a need for rigourous testing as well as regulating them accordingly. “There has been a preoccupation on concentrating on mechanisms of action that has resulted in many chemicals not being classified as endocrine disruptors,” Professor Demeneix tells.
Endocrine-disrupting chemicals can be found in flame retardants, toiletries, surfactants, plastics, textile products and also pesticides.
There are things that you can try to do in your everyday life to limit the amounts of such chemicals that you are exposed to. “Eating organic food can reduce your pesticide exposure. Also, you should air your home regularly and it is also very important to avoid smoking, either actively or passively, since this can alter thyroid hormone levels in early pregnancy,” she advises.
Thyroid hormone levels of mothers are crucial for brain development in the unborn child. Without the right amount at the right time, the chances of autism and reduced IQ increase.
Barbara Demeneix is a professor currently working at the French National Centre for Scientific Research (CNRS) at the National Museum of Natural History in Paris, France. Her research has focused on the evolution of thyroid hormone signalling and has sought to understand how thyroid hormones activate or repress gene activity in different tissues at various developmental states and in changing physiological conditions.
She is the author of ‘Toxic Cocktail: How chemical pollution is poisoning our brains’ (Oxford University Press, 2017) which explores in detail how children born to women exposed to thyroid-disrupting chemicals have lower IQs and more neurodevelopmental disorders including autism and ADHD, among others.
“How chemicals can affect the health of developing children”
Quoted from ECHA - https://newsletter.echa.europa.eu/home/-/newsletter/entry/how-chemicals-can-affect-the-health-of-developing-children