Bush’s Ailment Traced to Overactive Thyroid : Presidency: Doctors say the gland condition can be treated quickly and easily in most cases.

WASHINGTON — President Bush’s doctors said Tuesday they believe they have found the cause of his recent heart irregularity--a mildly overactive thyroid gland that is expected to be easily treatable.

In a briefing at Bethesda Naval Medical Center, Bush’s chief physician, Dr. Burton Lee, said he was “pleased by this turn of events” because it cleared up questions about the cause of Bush’s ailment and because such problems are “usually resolved within a short time.”

The diagnosis means that doctors have ruled out heart disease, stress, fatigue or excessive exercise as causes of Bush’s heart irregularity and, therefore, do not think the President will have to change his lifestyle. Bush “was elated” at the news, spokesman Marlin Fitzwater said.

A slight possibility exists, however, that a tumor could be the underlying cause of the thyroid problem. Although Bush’s doctors insisted the chances of that are small, the President will undergo a thyroid scan within the next several days to examine the gland for any sign of irregularity and to determine what treatment would be appropriate.

The thyroid is a small organ in the neck that secretes a hormone called thyroxine. Thyroxine regulates how fast tissues throughout the body burn sugar and produce energy.

Excessive production of thyroxine, a condition called hyperthyroidism, “is one of the most common endocrine disorders in medicine,” said USC endocrinologist Richard Horton. “The good news is that it explains the President’s problem, is easily treatable and essentially curable.”

Meanwhile, with his heart rate normal for a second day in a row, Bush began to relax Tuesday, offering a group of White House visitors a “thank you from the bottom of my formerly fibrillating heart” and showing reporters his portable heart monitor.

“Same old me,” Bush declared as he joked his way through a busy day’s schedule. “I’ve got to admit I’m glad to be out of the hospital,” he told one group. “It’s a little unsettling to turn on the news and see Peter Jennings pointing to a diagram of a heart with your name on it. It’s not even Valentine’s Day.”

The jokes reflected a far less tense atmosphere at the White House as the President continued to rebound from his weekend hospitalization.

After eight hours of sleep, Bush rose at dawn, had a complete--and normal--electrocardiogram and arrived at his office at 7:23 a.m. for a day packed with public appearances.

He did not, however, have his normal morning coffee. On the advice of his wife, the President has switched to decaffeinated coffee, his spokesman revealed. According to Mrs. Bush, “he says: ‘I barely have any,’ ” but “I’d say he has six for starters.”

Medical experts say there is little evidence that the caffeine in coffee can cause irregular heartbeats, but some recommend that patients avoid artificial stimulants to be on the safe side. The President “agreed with (Mrs. Bush) that prudence was a good course to follow,” Fitzwater said.

During the course of the day, Bush presented awards to a small-business group, discussed the situation in the Soviet Union over lunch with the president of Finland, met the president of Italy and showed reporters the wires on his heart monitor.

The monitor--a small device about the size of a telephone pager that Bush wears on his waist, picks up heartbeats from two sensors taped to his chest and relays them to a viewing screen located outside his office.

Bush declined, however, to show reporters the chest pads, themselves. “Do you think I’m Lyndon Johnson?” he asked, recalling the late President Johnson’s display to photographers of the scar from his gallbladder surgery.

But Bush’s visits to the hospital are not over. Now that doctors have determined the likely cause of his ailment, “it will take us at least a week to make sure we have all the tests we need,” Lee said.

Once the tests are finished, the President’s treatment will take one of several forms, said Dr. Kenneth Burman, an endocrinologist at Walter Reed Army Medical Center who has also treated Mrs. Bush for a thyroid disorder.

The simplest option would be merely to wait to see if the problem goes away by itself. Thyroid problems often are caused by inflammations that subside naturally.

Other possibilities include having Bush drink a radioactive iodine solution or take pills to counteract the excess hormones being produced by the thyroid. Surgery would be the most drastic response, but “the chances that President Bush is going to be operated on for this are basically nil,” Lee said.

Times science writer Thomas Maugh in Los Angeles contributed to this story.