Toxic Exposures
Hundreds of Studies Confirm: Endocrine-disruptors Harm Reproduction and Metabolism, and May Cause Thyroid Dysfunction
A comprehensive literature review in Endocrines, published in September, amasses hundreds of studies on endocrine-disrupting chemicals that showcase adverse effects on growth, development, reproduction and metabolism. The researchers also hed light on the link between the chemicals and thyroid dysfunction,
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A comprehensive literature review in Endocrines, published in September, amasses hundreds of studies on endocrine-disrupting chemicals (EDCs) that showcase adverse effects on growth, development, reproduction and metabolism.
The authors, a team of scientists and academics from Brazil, shed light on the link between EDC use and thyroid dysfunction, leading to increasingly prevalent illnesses and deadly diseases.
EDCs are any synthetic or natural compounds that hinder endocrine system functions and create harmful effects on organisms. These chemicals can impact the thyroid gland, which is vital in producing hormones and plays a role in several body functions.
As the researchers state, “Several EDCs have been classified as thyroid disruptors, impairing thyroid hormone [TH] production, synthesis, metabolism, transport, and/or actions.
Notably, thyroid disorders are the second most prevalent endocrine disease worldwide, with incidence increasing significantly in recent years.”
The authors continue:
“Studies have correlated this rise in thyroid dysfunctions and cancers with increased exposure to EDCs.
“Although many EDCs are linked to thyroid dysfunction, this review focuses on the deleterious effects of plasticizers, organochlorine pesticides, and per- and poly-fluoroalkyl substances on thyroid function. These contaminants are commonly found in food, water, and everyday products.”
This review helps to summarize the consequences of exposure to thyroid disruptors for human health, which can impact hormones such as diiodothyronine (T2), triiodothyronine (T3), thyroxine (T4) and the thyroid stimulating hormone (TSH).
These hormones are necessary for growth and metabolism, as well as fetal development and maturation, and play a key role in the central nervous system (CNS).
Several tissues in the body rely on these hormones, which highlights the importance of limiting exposure to the more than 100 natural and synthetic compounds that have been classified as thyroid disruptors. (See studies here and here.)
Impact on infants/children
Higher hazards of toxic chemical exposure are associated with infants and children, as their small size and developing organ systems make them more susceptible when exposed to toxic chemicals.
EDCs put young children at a disproportionate risk, “primarily because the metabolizing pathways of these substances are immature in these individuals,” the researchers point out.
“Moreover, embryos, fetuses, and newborns are adversely impacted by maternal exposure to these contaminants, which are transferred through the placenta and maternal milk. Consequently, these individuals become more vulnerable to hormonal, immunological, and neurological alterations during critical stages of their development.”
Additional studies indicate that maternal exposure to EDCs, both preconception and during pregnancy, can impact fetal development, birth outcomes and long-term health.
Some studies (see here, here and here) show that contact with EDCs “triggers epigenetic modifications in the placenta and the fetus, altering gene expression and compromising fetal development,” the authors note.
These changes can lead to subsequent health problems in the offspring and cause multigenerational and transgenerational effects.
Maternal THs are critical for fetal development, as the fetus receives them through the placenta exclusively up until weeks 12-16 of gestation. Any alterations in the mother’s hormone levels directly impact the fetus during this time. The transport of THs across the placenta aids in not only development but also in the function of placental tissues.
Studies have linked complications during pregnancy involving the placenta to untreated thyroid disorders, highlighting the role of TH production in birth outcomes. (See here and here.)
Plasticizers
Plasticizers, compounds utilized in the production of plastic products including phthalates and bisphenols, are frequently used due to their high stability.
These can be found in “various commercial products such as toys, construction materials, personal care products, cosmetics, food preservation films, detergents, and medical devices,” the authors share.
Exposure to phthalates, whether through ingestion, inhalation or dermal routes, can lead to detectable levels “in urine, blood, hair, amniotic fluid, and breast milk, confirming widespread human contamination of phthalates and their metabolites.”
Studies have not only detected these plasticizers in human samples, but have demonstrated an association between exposure, such as with di(2-ethylhexyl) phthalate (DEHP), and alterations in T3, T4 and TSH concentrations in pregnant women, newborns and children. (See studies here, here, here, here and here.)
Impacts within the general populations have also been noted, as well as later-in-life impacts after neonatal exposure to phthalates.
Bisphenol A (BPA), used in products like polycarbonate plastics and food can linings, has also been found in urine and other samples from adults and newborns and is linked to impacts on thyroid function and hormone levels.
Another study finds that elevated levels of bisphenols in urine correlate with increased levels of oxidative stress biomarkers and an enhanced risk of thyroid cancer.
The authors summarize these results by stating:
“[T]he data strongly indicate that BPA and its analogs are significant disruptors of thyroid function, with far-reaching implications for both human and other animals’ health. BPA’s ability to bind to TH receptors and thyroid transcription factors highlights its potent role in altering thyroid signaling pathways and gene expression.”
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Pesticides
Numerous pesticides are considered EDCs. Substances identified by the European Union (EU) include triclosan, atrazine, mancozeb, and propiconazole, among others.
Many EDCs have not been specifically tested for impacts on thyroid function, highlighting data gaps that warrant further research.
The Endocrine Society further emphasizes:
“There are nearly 85,000 man-made chemicals in the world, many of which people come into contact with every day. Only about one percent of them have been studied for safety; however, 1,000 or more of these chemicals may be EDCs based on their probable endocrine-interfering properties.”
A literature review in Frontiers in Endocrinology agrees in sharing from their research:
“We conclude first, that many pesticides were placed on the market with insufficient testing, other than acute or chronic toxicity, and second, that thyroid-specific endpoints for neurodevelopmental effects and mixture assessment are largely absent from regulatory directives.”
These authors also analyze classes of pesticides such as organochlorine pesticides (OCPs), organophosphates, carbamates, phenylpyrazoles and pyrethroids for thyroid disruption.
The research shows potential effects for many pesticides within these classes including dichlorodiphenyltrichloroethane (DDT), chlorpyrifos, thiram, fipronil and permethrin, respectively.
The original literature review, in Endocrines, also notes how OCPs (specifically DDT, endosulfan and dieldrin) are known for their persistence in the environment. DDT, for example, was widely used during World War II until it was banned in developed countries in the 1970s, but it is still detected in the environment and is used in several developing countries to this day.
Both DDT and its main metabolite dichlorodiphenyldichloroethylene (DDE) accumulate in the fatty tissue of humans and other animals after exposure, mainly through contaminated food and water.
As the researchers share, “DDT and DDE have a similar chemical structure to THs. For this reason, they are described as potential disruptors of TH synthesis, action, and metabolism.”
Studies find that DDT and its metabolites cross the placenta and are also detected in samples of breast milk. Reports show sexually dimorphic impacts from maternal and paternal exposure to DDT with impacts on placental genes, growth and function, and metabolic parameters in offspring.
“[T]he evidence shows that OCPs, particularly DDT and its metabolite DDE, disrupt thyroid function in both humans and other animals,” the authors affirm.
Per- and poly-fluoroalkyl substances (PFAS)
PFAS are a class of human-made chemicals of increasing concern that are also highly persistent and chemically stable.
“[M]ixtures of PFASs are commonly found in drinking water, and serum PFASs are detected in up to 99% of the population,” the researchers report.
Exposure to PFAS is negatively associated with TSH and free T4 levels, causing implications for early neurodevelopment. Prenatal exposure to PFAS is also negatively associated with the IQ of school-aged children.
Studies have detected higher levels of PFAS in infants with congenital hypothyroidism, which offers a link between PFAS exposure and thyroid function. Additional research highlights the potential for multi-transgenerational effects with PFAS as thyroid disruptors as well.
“Concerning thyroid cancer, PFAS exposure has been previously associated with an increased risk of papillary thyroid tumor development in humans,” said the authors.
They continue:
“The carcinogenesis associated with PFAS exposure has been related to changes in epigenetic mechanisms, immunosuppression, increased oxidative stress/inflammation, [and] alterations in hormonal/metabolic pathways.”
Summary of health implications
Through the review of a multitude of studies, the researchers find that EDCs alter thyroid function and hormone levels through several pathways. These changes impact cellular physiology and gene expression, which then leads to additional health implications.
“Exposure to phthalates, bisphenols, DDT, and PFASs has been linked to disruptions in the function of the hypothalamus–pituitary–thyroid axis, as well as in the cellular physiology and gene expression of thyrocytes,” the authors summarize.
Thyrocytes are the primary cells in the thyroid gland that produce and secrete thyroid hormones.
“Research indicates that these EDCs impair the activity and expression of proteins critical for thyroid hormone synthesis. Additionally, findings suggest that these EDCs possibly alter signaling pathways and epigenetic mechanisms that regulate gene expression in thyrocytes,” they conclude.
The studies analyzed showcase the associations between thyroid dysfunction and various conditions such as with cardiovascular, metabolic, reproductive, neurological, behavioral and fetal development disorders.
With these findings and the importance of TH throughout the entire body, the authors call attention to the chemicals that impair thyroid function and can have deleterious effects throughout generations.
History of EDC and thyroid function
The literature review in Endocrines is the latest addition to the scientific critiques showing the adverse effects of EDCs. Previous epidemiological studies have found an increase in the incidence and prevalence of diseases associated with exposure to EDC.
These include higher rates of breast, thyroid, prostate and testicular cancers, diabetes, obesity and decreased fertility over the past 50+ years. These effects occur when compounds, such as those in EDCs, bind to hormone receptors, trigger disruption in key pathways and disrupt hormone synthesis.
Global concerns for EDC have been noted, with the European Commission restricting known endocrine disruptors in 2023 as previously covered by Beyond Pesticides. The U.S. Environmental Protection Agency (EPA), however, has been under scrutiny for incomplete testing protocols for endocrine-disrupting pesticides.
As reported earlier this year, the EPA’s review of these chemicals ignores a range of chronic adverse health effects even at extremely low exposure levels. (See additional information on EPA failures pertaining to endocrine disruptors here and here.)
Other Beyond Pesticides’ coverage notes that the ingredients in many pesticides (and in many consumer products) act as endocrine disruptors in humans and other animals in several ways and urges advocates to take action in telling EPA to end the use of endocrine-disrupting pesticides.
This is due to EDCs ability to: (1) mimic actions of hormones the body produces (e.g., estrogen or testosterone), causing reactions similar to those generated by the naturally produced hormones; (2) block hormone receptor cells, thereby preventing the actions of natural hormones; and/or (3) affect the synthesis, transport, metabolism, and/or excretion of hormones, thus altering the concentrations of natural hormones in tissues or at receptor sites.
In 2021, Beyond Pesticides also reported:
“Specific to thyroid function, pesticides can inhibit iodine uptake, binding to hormone receptors and transport proteins, and interfere with gene expression. However, impacts on the thyroid are not the only result of endocrine disruption.
“The entire endocrine system directly affects traditional endocrine glands and their hormones and receptors (i.e., estrogens, anti-androgens, thyroid hormones).
“Furthermore, endocrine disruption can negatively impact reproductive function, nervous system function, metabolic/immune function, and fetal/body development.
“Considering endocrine disruption has such close ties with hormone-related cancers like thyroid cancer, it is essential to avoid toxic chemical exposure to lessen potential cancer risks.”
Organic as the solution
Beyond Pesticides’ mission is to provide healthy air, water, land and food for current and future generations by eliminating all petrochemical pesticides, including EDCs, and synthetic fertilizers by 2032. Beyond Pesticides also advocates for a precautionary approach to how land is managed, with organic agriculture as the answer.
You can support this holistic solution by educating yourself through the Daily News Blog and the Pesticide-Induced Diseases Database, as well as buying or growing organic food.
Make The Safer Choice regarding pesticides in your home and community, and sign up for Action of the Week and Weekly News Updates. For more information on why organic is the right choice for both consumers and the farmworkers who grow our food, see Health Benefits of Organic Agriculture.
Originally published by Beyond Pesticides.
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