What is the thyroid system?
Thyroid hormones perform a highly varied number of functions in many of the body’s organs and are an essential part of the body’s metabolic control system, regulating how quickly the body uses energy and governing the body’s sensitivity to other hormone signals.
One reason the possibility of chemical interference with the thyroid system is of particular interest to environmental health researchers is the key role thyroid hormones play in the development of the brain and nervous system, controlling synapse development, the growth of the myelin coating which sheathes nerve cells (essential for passing nerve signals), and proper functional arrangement of neurones in the brain.
How can disrupting the thyroid system affect health?
The mother provides all the thyroid hormone necessary for foetal development until relatively late in pregnancy. Even relatively minor fluctuations in levels of thyroid hormone in the mother’s blood during pregnancy can result in neurodevelopmental deficiencies in the foetus (Boas et al. 2006).
Children of mothers with low circulating thyroid hormone have been shown to have trouble with motor coordination, balance and other psychomotor skills. Attention deficit disorders and higher-than-average incidence of difficulties with spatial relations, perception, memory and language have been similarly linked.
Long-term low thyroid function in adults causes poor blood circulation. This reduces delivery of oxygen to body tissues and is associated with heart disease and cancer. Increased weight, depression, loss of hair and impaired cognitive function are other symptoms of low levels of thyroid hormones. There is evidence of generally reduced thyroid function in workers on primitive e-waste sites in China, thought to be due to common use of thyroid disruptors in electronic goods (Zhang et al. 2010).
Which chemicals can interfere with the thyroid system?
The thyroid system is complex and there are many ways in which chemicals can interfere with it, including inhibiting the thyroid gland’s ability to absorb iodine (phthalates and perchlorate), obstructing the enzymes which manufacture thyroid hormones (several pesticides and fungicides), and preventing thyroid hormones being activated or deactivated (lead, chromium, UV-blockers and the pesticide methoxychlor) (Patrick 2009).
Chemicals which are relatively well-researched with regard to thyroid-disrupting effects include PCBs, BPA, perchlorate, dioxins, pentachlorophenol (a breakdown product in the body of the pesticide hexachlorobenzene), triclosan and the PBDE flame retardants. Animal evidence of thyroid disruption exists for the phthalates DEHP, DnOP, DIDP, DNHP, DBP, resorcinol used in antiseptic creams and hair dyes and the flame retardant TBBPA, now the most commonly-used flame retardant in Europe (Zoeller 2010).
Environmental chemicals and thyroid function. Scientific review of animal evidence which suggests the potential for environmental chemicals to interfere with thyroid function ought to be of concern, particularly for its consequences for foetal neurological development.
Thyroid disruption: mechanism and clinical implications in human health. Accessible scientific review of research suggesting that chemicals may be affecting thyroid function.
Environmental chemicals targeting thyroid. Examination of the deficiencies in testing for the potential of chemicals to disrupt thyroid function, and the changes which need to be made to risk assessment as a result.
Selection of references
Boas M, Feldt-Rasmussen U, Skakkebaek NE, Main KM. Environmental chemicals and thyroid function. Eur J Endocrinol. 2006 May;154(5):599-611. Review.
Patrick L. Thyroid disruption: mechanism and clinical implications in human health. Altern Med Rev. 2009 Dec;14(4):326-46. Review. Erratum in: Altern Med Rev. 2010 Apr;15(1):58.
Zhang J, Jiang Y, Zhou J, Wu B, Liang Y, Peng Z, Fang D, Liu B, Huang H, He C, Wang C, Lu F. Elevated body burdens of PBDEs, dioxins, and PCBs on thyroid hormone homeostasis at an electronic waste recycling site in China. Environ Sci Technol. 2010 May 15;44(10):3956-62. PubMed PMID: 20408536.
Zoeller RT. Environmental chemicals targeting thyroid. Hormones (Athens). 2010 Jan-Mar;9(1):28-40.