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According to researchers here at the American Thyroid Association meeting on May 15, 2010, multiple genetic and environmental risk factors can trigger autoimmune thyroid disease in humans.
Professor of medicine and physiology at the David Geffen School of Medicine at the University of California, Gregory A. Brent, MD, showed the discovery. He said that “genetic factors accounts for about 70 percent of the risk for autoimmune thyroid disease. The rest is likely related to environmental triggers.”
Some of toxicant agents ruin iodine uptake and thyroid hormone synthesis, binding, metabolism, and action. He noted that among the more common agents that affect thyroid metabolism are thiocyanates (a compound in cigarettes), perchlorates (a rocket fuel additive), and dioxins (a class of pesticides).
Other risk factors for humans are environmental releases of radiation, environmental pollutants, medical radiation, and excess iodine intake. Medical radiation may trigger autoimmune thyroid disease and antithyroid antibodies.
“Toxicologists want to know the dose given. For these radiation accidents, we have the best evidence of the [disease] relationship because the dose is known,” Dr. Brent added. He showed examples from selected studies in humans that demonstrated the effects of I131 releases in nuclear accidents and after the atomic bomb.
One study that followed a cohort of atomic bomb survivors found that many people developed antithyroid antibodies and hypothyroidism. The disorders manifested a U-shaped relationship, as did findings in studies of the Chernobyl survivors.
Iodine added to the diet can trigger disease in select individuals and populations, according to Dr. Brent. He noted that among people in iodine-deficient areas, giving them iodine can stimulate immune response and antibodies.
Researchers have the least evidence for environmental toxicant effects in humans, Dr. Brent stated. “A whole range of compounds are being increasingly recognized that are associated with the impairment of thyroid hormone action,” he told meeting attendees.
The thyroid is influenced by toxicants. Researchers know very little about their effects in humans. These compounds include polychlorinated biphenyls (PCBs), pesticides, polybrominated diphenyl ethers (PBDEs), and bisphenol A (BPA).
In experimental animals, BPA has been shown to affect thyroid receptors and thyroid function, said Dr. Brent. Although BPA is of concern now, evidence of its effects in humans is sparse. Some evidence exists for industrial exposure to other compounds, but these lack dose-response data.
The way to avoid thyroid problems is to recognize risk factors and try to avoid them, Dr. Brent said. He noted that environmental factors account for about 30% of the risk for autoimmune thyroid disease. Risk factors that are easily avoided include certain drugs, cigarette smoke, stress (although difficult to prove), selenium deficiency, and contaminated well water (perchlorates).
