Thyroid Diseases and Glyphosate. What is the Relationship?

The increasing incidence of thyroid diseases raises a series of questions about the main predisposing factors currently at play. While dietary iodine restriction was once a major global health problem, today the industrialization of food processes and high exposure to a wide variety of environmental chemicals may be directly or indirectly affecting thyroid function.

Glyphosate is a highly effective, non-selective, post-emergent herbicide used to control harmful weeds in various crops. Glyphosate has been associated with various diseases, including thyroid disorders. However, regulatory agencies have not found any evidence to classify glyphosate as an endocrine disruptor or carcinogen. Despite this, there is evidence linking glyphosate exposure to the incidence of non-Hodgkin lymphoma, and the International Agency for Research on Cancer (IARC/WHO) has classified it as “probably carcinogenic to humans” (Group 2A).

In humans, three studies evaluated occupational pesticide exposure and suggested that GBH (glyphosate-based herbicides) may be involved in thyroid dysfunction. Kongtip et al. observed higher levels of TSH, T3, and T4 in conventional farmers compared to organic farmers, with glyphosate being the most used compound, followed by paraquat and 2,4-dichlorophenoxyacetic acid (2,4-D). In Brazil, researchers evaluated T3, T4, and TSH levels in soybean farmers who used pesticides, including glyphosate, and found a significant decrease in TSH and an increase in T3 and T4. Another study evaluated the association between the use of pesticides, including glyphosate, and the incidence of hypothyroidism in applicators over a 20-year period, and found that the risk of hypothyroidism significantly increased due to contact with these substances.

According to a 2021 review, certain points are very clear regarding the subject: ● The regulation of the HPT axis is very complex, with several stages that can be affected by xenobiotics. ● Glyphosate alters selenium absorption, the functioning of Ca++ channels, and disrupts oxidative phosphorylation, potentially affecting thyroid function. ● Oxidative stress is present in autoimmune thyroiditis and thyroid carcinomas. ● The HPT axis is recognized as a target for EDCs. ● Commercial formulations (GBH) are more toxic than glyphosate. ● Few thyroid toxicology studies have been conducted to date. ● In tests validated by regulatory bodies, the observed thyroid changes are disregarded, as allosteric regulation is observed. ● Glyphosate is detected in the urine of rural and urban residents. ● There is a correlation between farmers' exposure to GBHs and altered thyroid hormone levels or incidence of thyroid pathologies. It is concluded that few studies evaluate the mechanisms of toxicity (where, how, and if it occurs), which makes the extrapolation of correlations between human exposure and toxicity fragile, and that it may be time to start considering toxicity testing of commercial formulations, as they represent actual human and environmental exposure.

Reference: Romano RM, de Oliveira JM, de Oliveira VM, de Oliveira IM, Torres YR, Bargi-Souza P, Martino Andrade AJ, Romano MA. Could Glyphosate and Glyphosate-Based Herbicides Be Associated With Increased Thyroid Diseases Worldwide? Front Endocrinol (Lausanne). 2021 Mar 19;12:627167. doi: 10.3389/fendo.2021.627167. PMID: 33815286; PMCID: PMC8018287.

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