A federal magistrate ordered Tylenol-case suspect Leann Lewis held on a $5 million cash bond today because of a death threat against President Reagan that she allegedly helped her husband mail to the White House.

U.S. Attorney Dan Webb disclosed the letter in guarded fashion at a courtroom hearing this afternoon in pressing for a bond high enough to keep her in custody.

The note "alluded to the Tylenol tragedy" and threatened the life of Reagan and others, Webb said at a news conference. He added that the letter was "in the handwriting of Mr. [James W.] Lewis" and was sent in an envelope stamped with a postage meter to which Mrs. Lewis allegedly had access.

The threat against the president "related to a complaint Mr. Lewis had relating to certain policy positions taken by the White House," Webb said. "It did not demand any money."

Authorities said the postage meter used on the letter had been used in the $1 million extortion letter the Lewises allegedly sent to the manufacturer of Tylenol about Oct. 6.

That letter demanded $1 million from Johnson & Johnson "if you want to stop the killings" and was received by the company shortly after seven persons in the Chicago area died from swallowing Extra-Strength Tylenol capsules to which cyanide had been added.

"So far I have spent less than $50, and it takes me less than 10 minutes per bottle," the letter to Johnson & Johnson said. It was signed "Robert Richardson," a name that James Lewis used when he was living in the Chicago and New York City areas.

Officials would not say whether the letter threatening Reagan's life was dated but did say that it was mailed after the extortion letter and while the Lewises "were on fugitive status."

At the 15-minute hearing here, U.S. Magistrate James T. Balog was also told that Kansas City, Mo., authorities have come up with new evidence in the 1978 murder of a 72-year-old Kansas City man who had befriended the Lewises' daughter. Webb gave no details but said Kansas City police want to question the Lewises in light of the information.

Balog ordered Mrs. Lewis, 35, confined at the federal correctional center here and scheduled a preliminary hearing next Wednesday. The only charge against her is an allegation that she used a false Social Security number when she applied for a job with a Chicago area travel agency last Jan. 12.

Lewis was arrested in New York City Monday by FBI agents who found him in an annex of the New York Public Library in Manhattan.

Webb said Mrs. Lewis' lawyer, Michael Monaco, called him last night and said she "wanted to come to Chicago today and surrender." She arrived to O'Hare International Airport on a flight from Philadelphia and was taken into custody.

One of Webb's top prosecutors, Jeremy D. Margolis, will proceed with the federal charges against the Lewises without waiting for resolution of the Tylenol case, Webb said. He said the threat against Reagan "will, we anticipate, be part of the overall charges to be filed."