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    BOSTON (AP) _ Barbara Bush’s most recognizable feature, her silvery hair, may also be a sign of her most recent medical problem, an overactive thyroid gland, doctors say.

    Although no one knows exactly why, prematurely gray hair and an illness called Graves’ disease sometimes go together.

    Graves’ disease occurs when the thyroid glands in the neck churn out too much hormone, causing a variety of seemingly unrelated symptoms. The ailment is fairly rare but easily treatable with radioactive iodine, a therapy that Mrs. Bush received Wednesday.

    ″Many of us worried about her thyroid long ago when we heard about the premature graying of her hair,″ said Dr. Gilbert H. Daniels of Massachusetts General Hospital in Boston.

    Mrs. Bush’s hair turned white three decades ago, when she was in her early 30s. Untimely gray hair certainly isn’t harmful, but people who get it are also at higher than usual risk of Graves’ disease as well as several other conditions, including diabetes, pernicious anemia and white skin patches called vitiligo.

    Graves’ disease, which afflicts four in every 1,000 people, is an inherited disorder caused by a mixup in the body’s disease-fighting immune system. The body produces antibodies that mistakenly attack the thyroid gland as an enemy. Instead of harming the gland, they stimulate it to grow and work non-stop, producing too much thyroid hormone. Though the tendency is passed from parent to child, no one knows precisely what triggers the antibody attack to occur.

    The overabundance of this hormone, called thyroxine, speeds up the body’s chemical reactions. Like Mrs. Bush, victims may lose weight and have irritated eyes. Frequently they complain of gritty feeling eyes or blurred vision.

    Symptoms vary from person to person but can also include:

    -anxiety and fidgetiness. People may have trouble sleeping and their hands may tremble.

    -insensitivity to cold. Victims may feel comfortable in summer clothes on cold days and complain that rooms are too warm.

    -rapid heartbeat. The heart works harder than necessary, and this may be dangerous in people with underlying heart disease.

    -enlarged thyroid gland, a condition known as goiter.

    -protruding eyes that make people seem to be staring.

    ″In the untreated state, it can certainly be life-threatening, but these days we recognize and treat it very readily,″ said Dr. David Madoff of Johns Hopkins University.

    The most common treatment is radioactive iodine, which when introduced 50 years ago was one of the first medical uses of radioactive material.

    The thyroid gland needs iodine to make hormone and absorbs it readily. However, the gland cannot distinguish radioactive iodine from the normal variety.

    With this treatment, patients take a pill or drink radioactive iodine. The material is taken up by the thyroid gland, and over a period of months largely destroys it, leaving the gland shrunken and unable to make enough hormone. As a result, patients usually must take thyroid pills for the rest of their lives.

    ″It is simple, safe and effective,″ said Daniels, and does not cause cancer or other health problems.

    For a few days after treatment, however, people may give off slightly elevated amounts of radiation.

    ″You have to avoid close contact with people for the next week and flush the toilet bowl twice for the next three days,″ said Madoff, ″because iodine not absorbed by the thyroid is excreted in the urine.″

    Medicine and surgery are also used to treat the disease. While anti-thyroid drugs can make people feel better, the disease usually returns when people stop taking them. Surgery accomplishes the same result as radioactive iodine but requires anesthesia and can occasionally result in damage to the nearby parathyroid glands.

    Graves’ disease (named for Irish physician Robert James Graves, who described it in 1835) is about eight times more common in women than in men.