Chemical contamination and the thyroid
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REVIEW
Chemical contamination and the thyroid Leonidas H. Duntas
Received: 18 March 2014 / Accepted: 29 September 2014 / Published online: 8 October 2014 Springer Science+Business Media New York 2014
Abstract Industrial chemical contaminants have a variable impact on the hypothalamic-pituitary-thyroid axis, this depending both on their class and on confounding factors. Today, mounting evidence is pointing to the role of environmental factors, and specifically EDCs, in the current distressing upsurge in the incidence of thyroid disease. The unease is warranted. These substances, which are nowadays rife in our environments (including in foodstuffs), have been shown to interfere with thyroid hormone action, biosynthesis, and metabolism, resulting in disruption of tissue homeostasis and/or thyroid function. Importantly, based on the concept of the ‘‘nonmonotonic dose–response curve’’, the relationship between dose and effect has often been found to be nonlinear. Thus, small doses can induce unpredictable, adverse effects, one case being polychlorinated biphenyls (PCBs), of which congener(s) may centrally inhibit the hypothalamic-pituitary-thyroid axis, or dissociate thyroid receptor and selectively affect thyroid hormone signaling and action. This means that PCBs can act as agonists or antagonists at the receptor level, underlining the complexity of the interaction. This review highlights the multifold activity of chemicals demonstrated to cause thyroid disruption. It also represents a call to action among clinicians to undertake systematic monitoring of thyroid function and registering of the classes of EDs and additionally urges broader scientific collaborations to clarify these chemicals’ molecular mechanisms of action, substances whose prevalence in our environments is disrupting not only the thyroid but all life on earth.
L. H. Duntas (&) Unit of Endocrinology, Metabolism and Diabetes, Evgenidion Hospital, University of Athens, Papadiamantopoulou 20, 11520 Athens, Greece e-mail: [email protected]
Keywords Thyroid hormone receptor Endocrine disruptors Chemical pollution Thyroid TSH Polychlorinated biphenyls Bisphenol A
Introduction ‘‘Feeble though we may seem, we have the power to influence the course of our planet….’’ Colin Hiram Tudge.1 An endocrine-disruptor (ED) is defined by the U.S. Environmental Protection Agency (EPA) as ‘‘an exogenous agent that interferes with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process’’ [1, 2]. This comprises a highly heterogeneous group including synthetic chemicals and their byproducts, plastics, plasticizers, pesticides, and fungicides. In addition, natural chemicals found in human and animal food such as phytoestrogens, as well as thyroid drugs, e.g., thyrostatics and, for non-thyroidal illness amiodarone, can all exert endocrine disrupting activity. Our planet as a whole is increasingly exposed to
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