The hormones produced by the thyroid gland are involved in the development of the fetus: a disturbance of the thyroid function in a pregnant woman is likely to have important consequences on the health of the unborn child. Exposure to certain environmental pollutants, in particular endocrine disruptors, can have an impact on thyroid function. To better understand this impact, an international consortium coordinated by researchers from Inserm, CNRS and the University of Grenoble Alpes conducted assays in the biological samples of more than 400 pregnant women and found an association between exposure to three commonly used pollutants (propyl-paraben, bisphenol A and butyl-benzyl phthalate) and abnormal levels of thyroid hormones. This work, to be published in Environmental Health Perspectives, alerts us to the presence of these endocrine disruptors in a majority of the participants and calls for more knowledge about their effects on pregnancy and the health of the unborn child.
The thyroid is a small gland located at the base of the neck that produces two hormones: triiodothyronine (or T3) and thyroxine (or T4); T3 is actually the product of the conversion of T4 to a more active form. Their secretion is regulated by another hormone, thyroid-stimulating hormone (or TSH) produced in the brain by another gland, the pituitary. These hormones are crucial for the development of the fetus; therefore, thyroid dysfunction in a pregnant woman is likely to impact the health of the unborn child. Several types of environmental factors are known to affect thyroid function, in particular exposure to pollutants such as endocrine disruptors.
An international research consortium coordinated by Claire Philippat, an Inserm researcher at the Institute for the Advancement of Biosciences (Inserm/CNRS/Grenoble Alpes University), in collaboration with the Grenoble Alpes University Hospital, studied the impact of exposure to molecules from the family of phenols, parabens and phthalates, considered to be chemical pollutants, on the thyroid hormone levels of pregnant women.
The researchers first identified several chemical pollutants that could affect thyroid function using a database of in vitro toxicology test results. The research team then worked with biological samples from 437 pregnant women from the SEPAGES cohort in Grenoble. By comparing the presence of pollutants in the urine and thyroid hormone concentrations in the blood, the scientists were able to study their potential associations.
Several chemical pollutants were detected in the majority of the urine samples collected, confirming that the participants were almost all exposed to them. A certain number of these molecules would also have a negative impact on thyroid function: exposure to propyl-paraben (a compound used as a preservative in the cosmetics, food and pharmaceutical industries) was associated with a decrease in T3 concentrations, while butyl-benzyl phthalate (used in particular in PVC-type plastics) was associated with an increase in T4 concentrations. In addition, the conversion of T4 to T3 also appeared to be affected. In both cases, these mechanisms resulted in abnormal proportions between the two hormones.
Another compound, bisphenol A (used, among others, in the manufacture of plastics), appeared to be associated with a decrease in TSH concentration.
Data from in vitro toxicological tests suggest that these pollutants could act on the mechanisms governing the synthesis and degradation of thyroid hormones," explains Claire Philippat. In particular, butyl-benzyl phthalate and bisphenol A could inhibit the incorporation of iodine - an essential element for the synthesis of thyroid hormones - into thyroid cells," she adds.
She continues: " This work reinforces the knowledge on the deleterious effects of exposure to certain chemical pollutants on thyroid function. In particular, it alerts us to the frequent exposure of the population to these pollutants and invites further research on the consequences for the health of the child, as even small variations in the mother's thyroid hormone levels during pregnancy can impact the fetus and its development."
Based on these findings, the researcher and her team are now interested in the potential impacts of these alterations in thyroid function on neurodevelopment and growth in children.