Environmental Factors and Autoimmune Thyroid Disease

Environmental Factors and Autoimmune Thyroid Disease

by W. Reid Litchfield, MD FACE

Autoimmune thyroid disease has generally been considered to be a familial syndrome, but environmental factors can be a significant contributor to the development of these disorders.

Autoimmune thyroid disease represents a spectrum of thyroid disorders. Included in this broad category of disorders is Hashimoto’s disease, Graves’ disease, nodular thyroid disease and the presence of anti-thyroid antibodies.

The role of genetics has long been recognized as very important in causing thyroid disease. In fact, genetics are estimated to account for about 70% of autoimmune thyroid disease. Individuals in an affected family may have a variety of thyroid problems ranging the full spectrum of diseases in a single family. For example, a parent may have Graves’ disease, and her children Hashimoto’s disease and or thyroid nodules.

Environmental factors probably account for about 30% of autoimmune thyroid diseases. There are a wide variety of environmental triggers. A number of chemicals have been shown to affect thyroid hormone production and thyroid function. These include perchlorates (an additive in rocket fuels), thiocyanates (found in cigarette smoke), and dioxins (found in pesticides). Unfortunately we have very few scientific studies looking at the extent to which these compounds affect the thyroid in humans. Additionaly, toxic chemicals are potentially implicated in thyroid disease. Potential toxicants include pesticides, polychlorinated biphenyls (PCB’s), polybrominated diphenyl esthers (PBE’s) and bisphenol A (BPA). These chemicals are increasingly present in our environment and may have implications in causing thyroid disease in humans. Unfortunately data in humans is still lacking and more research in this area is required.

Iodine is a known trigger of autoimmune thyroid disease. Iodine supplementation can trigger autoimmune thyroid disease and antibody formation and has been demonstrated when supplementation is initiated in iodine-deficient areas. Selenium deficiency has been associated with autoimmune thyroid disease, and its replacement has been demonstrated to improve anti-thyroid antibodies in several clinical trials.

Exposure to ionizing radiation is a well-known risk factor for thyroid cancer, but can also induce thyroid autoimmunity. Medical radiation (generally for cancer treatment) and radioactive fallout (Chernobyl nuclear accident, Hiroshima, nuclear testing) have all been linked to the development of anti-thyroid auto-antibodies and hypothyroidism. Radioactive iodine therapy has also been shown to cause the development of anti-thyroid antibodies and has been reported to cause Graves disease in some cases.

It is certainly likely that other undetermined environmental factors are also implicated in the development of autoimmune thyroid disease. The way in which all of these factors trigger the autoimmune process is not completely understood. Furthermore the way these triggers interact with things that seem to precipitate thyroid problems (such as stress, hormonal changes, etc.) needs additional study.

Until the environmental factors that can trigger thyroid disease are more clearly understood, we are left to take a common-sense approach to try and maintain good thyroid health. This includes adequate (but not excessive) intake of iodine and selenium, limiting exposure to ionizing radiation, smoking cessation, and avoidance of contamination by environmental toxicants through direct contact or by ingesting tainted drinking water.

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