Declaring himself on his way "back to normal," President Bush drank a dose of radioactive iodine Wednesday to begin the process of treating an overactive thyroid blamed for recent irregularities in his heartbeat.

Bush also said Wednesday at a news conference that Vice President Dan Quayle "has gotten a bum rap" and vowed that he will not change his mind about keeping Quayle as his running mate.Bush said he told Quayle to "keep your head up" and weather the criticism.

Bush spent a brief 15 minutes at Bethesda Naval Hospital as doctors administered the iodine, preparatory to tests Thursday morning that should help them assess the extent of the problem and prescribe treatment.

The upbeat president, clearly buoyed by the diagnosis, told visitors later in the Oval Office: "I go out tomorrow and take the medicine, whatever I have to do, and that takes care of it. Back to normal."

At the same time, however, White House spokesman Marlin Fitzwater disclosed the president had suffered another bout of high, erratic heartbeat about 6 p.m. Tuesday, despite treatment with two drugs designed to keep his heart rhythm normal.

The irregularity, or arrhythmia - Bush's first recurrence since being hospitalized over the weekend - was corrected within "a few minutes," Fitzwater said, and Bush's heart "quickly returned to normal rhythm."

But doctors took the additional step of prescribing an anti-coagulant, Coumadin, to avoid any possibility of a blood clot developing.

The new drug was administered "as a precaution against any coagulation problem that might occur," Fitzwater said.

The brief relapse, which was not wholly unexpected, was discovered on a heart monitor, to which Bush remains linked by portable transmitter 24 hours a day.

The thyroid ailment was identified Tuesday evening as the cause of the heart problem Bush suffered Saturday while jogging at Camp David, known as atrial fibrillation. The cause of the condition, however, remains unknown.

White House physician Burton Lee said tests will be made to further diagnose the problem and determine therapy. He emphasized, however, that the hyperthyroidism, a condition that also has afflicted Bush's wife, Barbara, should be easy to treat, eliminating concerns about his heart.

Bush expressed relief at a morning news conference that the cause of the heart abnormality had been pinpointed and could be cured.

Because of "the thyroid thing" doctors say he may have had for up to 12 weeks, he said he has been told for the time being to "not have as active an athletic regime as I'd like to."

"But they are elated that they know what caused this fibrillation and that it's curable and will be cured very soon," Bush said. "That's the good news. Once the thyroid is corrected, that means there's no problem on the heart - thyroid connected to the heart bone, you know. And they've assured me that it can be OK."

Treatment of the thyroid problem could involve medication or ingestion of radioactive iodine to kill the gland. Another possible but less likely course of action is surgically removing the thyroid.

The iodine, which is considered very safe, either reduces the thyroid's activity or destroys the butterfly shaped gland located in the front of the neck. If it is destroyed, Bush would have to take medication to replace the hormones the gland normally produces.

Reporters Wednesday asked Bush about Quayle, who has come under renewed scrutiny in light of Bush's irregular heartbeat. Several public opinion polls have showed that Quayle is widely considered unqualified to take over as president.

"I think he's getting a bum rap in the press, pounding on him when he's doing a first-class job," the president said. "And I don't know how many times I have to say it, but I'm not about to change my mind when I see his performance and know what he does."