A Fed spokesperson would not comment on whether Yellen planned to see a doctor for a follow up check-up, and would not comment on her health beyond the statement that she had felt dehydrated Thursday night after speaking for nearly an hour at the University of Massachusetts, Amherst.

Yellen, 69, who received medical assistance in a private room offstage, later attended a dinner with the university's chancellor and other special guests. She appeared fine and was talkative, according to one person at the dinner. A Fed spokesperson said Yellen felt fine after the incident.

Yellen is the most powerful figure in world finance and her health scare came at a key time for the Fed, which has been debating raising interest rates for the first time in nearly a decade. The speech was Yellen's first since last week's Fed decision to delay a much-anticipated rate "lift-off."

After the speech, during which Yellen appeared to lose focus, repeating some words and slowing down, the Fed chief was seen by the university's emergency team and later carried on with her schedule.

"If you ask me, is it concerning, I think it is concerning," said Dr. Andrew Stemer, an assistant professor of neurology and radiology at Georgetown University School of Medicine.

But he added it was hard to offer a diagnosis without lab tests, an exam or medical history: "I want to be cautious about speculating on things that I can't know...I'm sure she's getting excellent care and that her physicians are working this up."

U.S. presidents typically receive more exhaustive checks.

In 2014 President Barack Obama was admitted to hospital after a persistent sore throat and had a CT scan. President George W. Bush, who fainted in 2002 after choking on a pretzel, was checked out by a White House nurse and doctor, while his father was cleared by two assigned physicians to carry on with a Japan trip in 1992 after throwing up at a Tokyo dinner.

Fed chairs typically do not travel with medical staff. Instead they are accompanied by a security detail and at least one public relations handler, as Yellen did in Amherst on Thursday and Friday. Security officials conducted a 20-minute sweep of the university auditorium before the event, and they were on hand as she departed later.

St. Louis Fed President James Bullard said the central bank was well prepared in case its chief for some reason could not carry on performing her or his duties.

The Fed "has a lot of talented people so we would be able to carry on very well if there were serious problems," Bullard told reporters on Friday. "This was reviewed very carefully after 9/11 and we started getting better as an organization and we reviewed that thoroughly and those plans are basically in place."

Were a Fed chair to become incapacitated, the vice chair — former Bank of Israel chief Stanley Fischer — would take over the Board of Governors. Depending on how long the Fed chair was away, William Dudley, head of the New York Fed and vice chair of the Federal Open Market Committee, would temporarily chair the policy-setting committee pending a vote of the committee.

"It might be a little nerve-wracking for financial markets if that were to occur, but I think the fact that Fischer is well-known and well-respected could limit how much volatility a succession event would cause," said Michael Feroli, JPMorgan's chief U.S. economist and a former Fed staffer.

"Yellen and Fischer have been reared in the same intellectual tradition, so the differences in leadership should not be too large."