Hormones secreted by the thyroid gland control metabolism, or the speed at which the body converts food into energy. Metabolism is directly linked to the amount of hormones that circulate in the bloodstream. If, for some reason, the thyroid gland secretes an overabundance of these hormones, the body's metabolism goes into high gear, producing the pounding heart, sweating, trembling, and weight loss typically experienced by hyperthyroid people. Normally, the thyroid gets its production orders through another chemical called thyroid-stimulating hormone (TSH), released by the pituitary gland in the brain. But in Graves' disease, a malfunction in the body's immune system releases abnormal antibodies that mimic TSH. Spurred by these false signals to produce, the thyroid's hormone factories work overtime and exceed their normal quota.

Exactly why the immune system begins to produce these aberrant antibodies is unclear. Heredity and other characteristics seem to play a role in determining susceptibility. Studies show, for example, that if one identical twin contracts Graves' disease, there is a 20% likelihood that the other twin will get it, too. Also, women are more likely than men to develop the disease. And smokers who develop Graves' disease are more prone to eye problems than nonsmokers with the disease. No single gene causes Graves’ disease. It is thought to be triggered by both genetics and environmental factors, such as stress.

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