President Bush's doctors announced yesterday that he has Graves' disease, the same thyroid disorder that has affected his wife since 1989. They said that the president received a dose of radioactive iodine yesterday morning to bring his overactive thyroid gland under control.

Bush drank the iodine at Bethesda Naval Medical Center, then returned by helicopter to the White House, where doctors continued to monitor his heart electronically in case of a further recurrence of the abnormal heartbeat for which he was hospitalized last weekend.

The disease affects about 1 percent of Americans at some point in their lives, said Gilbert H. Daniels, co-director of the thyroid clinic at Massachusetts General Hospital. It is not life threatening if diagnosed and treated properly.

The treatment could result in a drop in the president's energy level, experts said.

The president's physician said efforts are being made to reduce Bush's workload over the next week. White House spokesman Marlin Fitzwater said the president will make fewer public appearances than planned and that Vice President Quayle will substitute for the president on a scheduled trip to Chicago next Monday.

The thyroid gland produces a hormone that regulates metabolism. With Graves' disease, the gland enlarges because it receives false messages from elsewhere in the body about how much hormone is needed. Doctors announced Tuesday that excessive levels of thyroid hormone had caused the president's atrial fibrillation.

The president's heartbeat has been normal except for a "rather brief episode" Tuesday night of atrial fibrillation, the abnormal rhythm that was first discovered last Saturday, said Bruce K. Lloyd, chief of cardiology at the naval hospital.

Bush's doctors said at a news conference yesterday morning that once the president's thyroid hormone level falls to normal, the risk of atrial fibrillation is expected to disappear, and Bush will then no longer need to take heart medications. He currently is receiving digoxin and procainamide, two cardiac drugs, as well as aspirin and Coumadin, a drug that prevents blood clots.

Allan Ross, chief of cardiology at George Washington University Medical Center, said White House doctors probably would remove the heart monitor within 12 to 24 hours.

Burton Lee, the president's physician, said Bush was "bothered" by the initial uncertainty over what had caused his heart problem, and that he was "very gratified by finding out that we had determined a cause."

Lee said Bush's doctors believe the thyroid condition has developed since his last physical examination six weeks ago. But he said Bush's thyroid had never been tested during previous checkups.

Kenneth Burman, a thyroid specialist at Walter Reed Army Medical Center who is treating the president, said there is a controversy over whether thyroid tests should be done routinely as part of physical examinations. He added that he does not consider them cost-effective. However, several doctors not connected with the case expressed surprise that the president had not previously had his thyroid tested.

Tests the president underwent during the last two days identified Graves' disease as the reason for his abnormally high levels of thyroid hormone. In one key test, Bush swallowed a tiny dose of radioactive iodine, a substance that collects in the thyroid gland, which is located in the front of the neck. Doctors then used an instrument like a Geiger counter to obtain an image of the walnut-sized gland.

This thyroid scan showed that Bush's thyroid was mildly but uniformly enlarged and that all of it was taking up iodine at a greater rate than normal. Experts said those findings are characteristic of Graves' disease, and virtually rule out the other two common causes of an overactive thyroid -- a temporary inflammation called thyroiditis, or a benign thyroid tumor or nodule.

Graves' disease is an immune system disorder in which the body makes a type of chemical antibody which mimics the hormone that normally tells the thyroid how much to produce. "It fools the thyroid into thinking it's receiving a normal stimulation," said Bruce D. Weintraub, a thyroid specialist at the National Institutes of Health.

In some Graves' disease patients, an antibody apparently triggers abnormal deposits of fat behind the eyes, creating a pop-eyed appearance and sometimes causing blurry vision.

Thyroid clinic co-director Daniels said the chances that both members of a married couple, such as the Bushes, would develop Graves' disease were only 1 in 10,000.

Experts said it is unlikely that any environmental factor could have triggered Graves' disease in both the president and the First Lady. "He's left-handed," said Daniels. "People with left-handedness are more prone to auto-immune thyroid disorders." He added that people whose hair turns prematurely gray -- as did the First Lady's -- are also at increased risk.

Burman said the president will take a saturated solution of iodine for about two weeks to bring his thyroid hormone levels down to normal more quickly. The radioactive iodine dose administered yesterday will make Bush mildly radioactive for a few days, and Lee said the president has been advised not to hug his grandchildren for the time being.

The radioactivity will destroy much of the gland, and the president's doctors said that within two or three months his hormone levels may drop so low that he will need to take a daily dose of thyroid hormone. Experts said Bush's personality is unlikely to change, but his energy level may drop a notch.

The American public may see "a slower and less frenetic George Bush," predicted Weintraub.