[Thyroid and the environment]

Abstract

It has long been known that the thyroid depends upon the environment for regular iodine supply, avoiding iodine deficiency or excess. Thyroid function may be altered by natural compounds present in water or foodstuff (such as iodine or phyto-goitrogens), or by synthetic compounds, either administered knowingly (in case of medicine), or as an untoward event in case of exposure to industrial products and pesticides, massively produced and polluting the environment. Compounds with an impact on thyroid homeostasis are called thyroid disruptors (TD). TD may disrupt the thyroid economy at any level of regulation: thyroid hormone synthesis, metabolism, or transport; cellular level including thyroid hormone signaling; tumorigenesis or more indirectly via the triggering of an autoimmune process. Compounds such as polychlorinated biphenyls (PCBs) may act at multiple levels. PT effects on human health depend on parameters linked to the individual person (age at exposure, iodine status, diet, professional exposure, place of living, family history of thyroid disease, detoxification enzyme genetic variants) and on parameters linked to the compounds themselves (chemical structure, lipo- or hydro-solubility, modes of exposure, metabolites activity, "cocktail effect"). The toxic effects of TD do not necessarily follow the rules of classical toxicology (low-dose effects, non-monotonic curves). The main clinical risks are the deleterious impact on neurocognition and behavior for the fetus and the young child, and possibly the elderly, while in adults the main concerns are tumori/goitrogenesis and autoimmune thyroid disease. The potential socioeconomic impact for society warrants an active and major involvement in research to find solutions in a multidisciplinary approach.