Environmental Endocrinology and the Hypothalamus-Pituitary-Thyroid Axis

Abstract

The hypothalamus represents a potential target for a wide spectrum of endocrine-disrupting chemicals (EDCs). Their effects can be exerted at a lower dose which may not be predicted at higher doses, there being a nonlinear relationship (nonmonotonic dose-response) between dose and effect. Among the various categories of EDCs that may disrupt thyroid function, some are more analytically discussed. Polychlorinated biphenyls (PCBs), which are halogenated organochlorines, show variable biodegradation depending on various congeners which may interfere at the different levels of thyroid hormone (TH) production, transportation, and metabolism; they may also display agonist or antagonist action by binding to TH receptor and affecting TH signaling. Pesticides, which are associated with low T4 and T3 levels, appear to be involved in the development of autoimmune thyroiditis. Plasticizers, like bisphenol A (BPA), widely used in consumer products, may perturb thyroid function altering the hypothalamic-pituitary thyroid axis when the fetus is exposed to these compounds prenatally. Heavy metals, even at low concentrations, can interfere with thyroid function by decreasing iodine uptake and accelerating thyroid parenchymal transformation, and, depending on duration of exposure, they may ultimately induce hypothyroidism. As stated by the WHO in 2010, “Humans are actively squandering and destroying nature’s wealth and abundance” (Gristle: From Factory Farms to Food Safety (Thinking Twice About the Meat We Eat) – Moby & Miyun Park (editors) – March 2010), thus what is needed is an entire reevaluation of our relationship with nature – and this obviously includes drastic reduction of the present-day tsunami of EDCs in our world.