Environmental factors

The hypothalamus–pituitary–thyroid–periphery (HPTP) axis has been known to be a vulnerable target for environmental factors and nutritional agents for centuries. Goitrogenesis, hypo- and hyperthyroidism, tumorigenesis, and autoimmune diseases of this gland have been linked to single or combined deficiencies of several essential trace elements. Normal thyroid function depends on adequate and balanced availability of the essential trace elements iodine, selenium, iron, and the mineral zinc in the daily diet. It has been suggested that the evolution of humankind and Eve’s route of migration out of Africa, to displace the Neanderthal people and to populate the other continents, closely followed coastlines and regions with high availability of iodine, the key element required for thyroid hormone synthesis (1, 2). Involuntary or voluntary environmental or nutritional exposure to adverse factors and agents impairing thyroid hormone synthesis, secretion, binding, transport, metabolism, and action (‘goitrogens’) contributes to the development and persistence of thyroid disorders (3). Iodine deficiency, still prevalent in many regions of our world, and iodine excess (4), both of which might occur during embryonal and fetal development as well as in newborns, adolescents, and adults, provide the platform for action of adverse agents, which might be well tolerated by a normally functioning ‘quiescent’ thyroid gland with adequate iodine supply (see Chapters 3.2.3, 3.2.4). Compounds adversely affecting the HPTP axis belong to several chemical classes of food ingredients and environmental contaminants, but might also represent pharmaceutical drugs acting either directly on biomolecules comprising the HPTP axis or after modification by phase I and/or II drug metabolism (see Table 3.2.2.1). Apart from by ingestion, several agents reach their targets after inhalation (e.g. occupational exposure or smoking) or by dermal application (e.g. UV screens).