BOSTON -- Red Sox slugger David Ortiz has been admitted to Massachusetts General Hospital for tests for a possible heart condition.

NewsCenter 5's Gail Huff reported that he arrived back in Boston late Tuesday night. He will been seen by a team internist, after a second bout with heart palpitations.

Doctors said that heart palpitations can occur for many reasons, including exercise, heart disease, anxiety or fatigue, or even an over-active thyroid.

It can be a sign of atrial fibrillation, which is treated with medication. Larry Bird has the condition. It can also signal something more serious.

This year, the NBA recommends that all players have echocardiograms and stress tests.

"There's something about the physiological tone that makes athletes more prone to these irregular heart beats," Tufts New England Medical Center Dr. Mark Link said.

Ortiz was admitted to Massachusetts General Hospital on Aug. 19 for a battery of tests. He said that it was stress-related, but with the second episode, doctors said that more tests are needed.

"All I know is that it could be serious and I think you have to be safe," former Red Sox player Dennis Eckersley said.

"Any time you are dealing with a medical condition like that, you hope for the best, and you hope that whatever tests they do will rule out anything serious," Red Sox player Mark Loretta said.