27
May
Exposure to Certain Pesticides Increase the Risk of Thyroid Cancer
(Beyond Pesticides, May 27, 2021) Research by the U.S. National Institutes of Health (NIH) finds exposure to lindane and metalaxyl pesticides heightens thyroid cancer risk. Both incidents of non-aggressive thyroid tumors and advanced-stage thyroid cancer are on the rise. However, researchers speculate that environmental pollutants, such as pesticides, may contribute to this increase, especially considering the pervasiveness of pesticide exposure among the general population.
Globally, cancer is one of the leading causes of death, with over 8 million people succumbing to the disease every year. Notably, the International Agency for Cancer Research (IARC) predicts new cancer cases to rise 67.4% by 2030. Various environmental pollutants like pesticides have endocrine (hormone) disruption effects that promote higher instances of thyroid and reproductive cancers. Therefore, studies like these highlight the importance of understanding how pesticide use can increase the risk of latent diseases (e.g., cancers), which do not readily develop upon initial exposure. The researchers state, “More work is needed to understand the potential role of these chemicals in thyroid carcinogenesis.”
The European Union and endocrine disruptor expert (deceased) Theo Colborn, Ph.D., classify more than 55 pesticide active ingredients as endocrine disruptors (EDs), including chemicals in household products like detergents, disinfectants, plastics, and pesticides. Endocrine disruptors are xenobiotics (i.e., toxic chemical substances foreign to an organism or ecosystem). Past systematic research shows exposure to endocrine-disrupting pesticides can adversely impact thyroid hormone disruption and disease development. However, this study is the first to evaluate associations between occupational exposure to specific pesticides and the risk of thyroid cancer development.
Researchers gathered information on pesticide use and exposure via the Agricultural Health Study, a cohort of licensed pesticide applicators (mainly male) between 1993 to 1997 and 1999 to 2005. Exposure reports include 50 different pesticides, 44 of which researchers evaluated exposure for thyroid cancer risk. Using cumulative intensity-weighted lifetime exposure, scientists determined factors influence exposure measured in days. Researchers assessed thyroid cancer incidents among male participants during 2014/2015 and estimated hazard ratios and confidence intervals using Cox regression.
The study finds participants’ exposure to metalaxyl (benzenoid fungicide) and lindane (organochlorine insecticide) increases the risk of thyroid cancer. Exposure to high levels of carbaryl (carbamate insecticide) has an inverse association with thyroid cancer. Additionally, chlorimuron-ethyl (herbicide) has an inverse association with a common thyroid cancer subtype, papillary cancer.
It is no secret that specific endocrine-disrupting chemicals can induce changes in thyroid function, including pesticides like organochlorine, organophosphate, carbamate, and pyrethroid. 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. NIH’s National Cancer Institute also finds many cancer-causing substances are endocrine disruptors. Moreover, 66 percent of all cancers have links to environmental factors, especially in occupations of high chemical use. 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.
This study is the first to show a direct association between thyroid cancer—rather than function or disease—and specific occupational pesticides. Organochlorine pesticides (OCPs) are well-known persistent organic pollutants (POPs) banned by the Stockholm Convention treaty in 2001. OCPs, like lindane, are primary pollutants of concern (UNEP, 2009), as their persistence and toxicity have adverse effects on environmental and biological health. These pollutants have a global distribution, with evaporation and precipitation facilitating long-range atmospheric transport, deposition, and bioaccumulation of hazardous chemicals in the environment. The U.S. was a signatory to the treaty, but U.S. Senate never ratified it, relegating U.S. officials to observer status.
While various POPs on the Stockholm Convention annex lists are no longer manufactured or utilized, lindane is an exemption, remaining active in pharmaceutical lice and scabies treatments. Metalaxyl is not a POP or banned chemical; however, extensive use of this fungicide has resulted in the primary fungi target (Pythium spp.) developing severe resistance. Additionally, a 2013 report found 117 pesticides, including metalaxyl, present in Long Island, New York’s drinking water. The chemical readily leaches, is highly soluble and persistent in water, and has links to acute toxicity, kidney and liver damage, and toxicity to birds. Metalaxyl alone was present 1,327 times in 546 different locations on Long Island. Lindane and metalaxyl can work together, or synergize, to produce a more severe, combined effect. Synergism is a common issue among pesticide mixtures and can underestimate the toxic impacts on human, animal, and environmental health. Hence, individuals encountering both chemicals, from pharmaceutical treatments and overapplication of chemicals to compensate for resistance, face elevated health risks.
The connection between pesticides and associated cancer risks is nothing new. Several studies link pesticide use and residue to various cancers, from more prevalent forms, like breast cancer, to rare forms like kidney cancer nephroblastoma (Wilms’ tumor). A past report using the same Agricultural Health Study (AHS) cohort demonstrates an association between non-Hodgkin’s lymphoma and lindane, establishing the chemical as carcinogenic to humans. Moreover, the same AHS cohort displayed higher thyroid cancer rates than the general population, especially regarding past use of atrazine (herbicide) and malathion (insecticide). Although this study finds that carbaryl has an inverse association with thyroid cancer, the U.S. Environmental Protection Agency (EPA) classifies the chemical as “likely to be carcinogenic to humans.” Furthermore, the European Commission report on endocrine disrupting substances classifies carbaryl as a category one endocrine disruptor, the highest category ranking.
Despite the rarity of endocrine cancers, thyroid cancer is the most common type, occurring before 40 years old in >30 percent of patients. Furthermore, women are at a higher risk for thyroid cancer than men, and pesticide exposures can disproportionately impact one sex more than the other. Thus, study researchers advocate for the evaluation of thyroid cancer risk and chemical exposure between sexes.
There is a lack of understanding behind the etiology of pesticide-induced diseases, including predictable lag time between chemical exposure, health impacts, and epidemiological data. Exposure to pesticides can increase the risk of developing chronic illnesses that may be rare and disproportionately impact various populations. Therefore, studies related to pesticides and cancer can help scientists understand the underlying mechanisms that cause the disease.
Beyond Pesticides tracks the most recent studies related to pesticide exposure through our Pesticide Induced Diseases Database (PIDD). This database supports the clear need for strategic action to shift away from pesticide dependency. For more information on the multiple harms pesticides pose on human health, see PIDD pages on cancer (including thyroid), endocrine disruption, and other diseases.
Beyond Pesticides advocates a precautionary approach to pest management in land management and agriculture by transitioning to organic. Buying, growing, and supporting organic can help eliminate the extensive use of pesticides in the environment and from your diet. For more information on why organic is the right choice for both consumers and the farmworkers who grow our food, see Beyond Pesticides webpage, Health Benefits of Organic Agriculture.
Help Beyond Pesticides educate and build the movement that will bring long-needed protection to humans, animals, and the entire environment by attending the National Pesticide Forum June 1, 8 and 15. Cultivating Healthy Communities brings together expert scientists, farmers, policymakers, and activists to discuss strategies to eliminate harms from toxic chemical use in favor of nontoxic organic solutions. The conference began with a pre-conference on May 24, launched on May 125, and continues every Tuesday beginning June 1, June 8, and ending June 15, 2021. Registration is open today and available through the webpage on this link. It starts with us. Upon registration, you will be able to view talks from the entire conference.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: Oncology Learning Network, Environment International
