Secretary of State Hillary Clinton has been admitted to a hospital with a blood clot following a concussion she suffered several weeks ago.
A spokesperson, Philippe Reines, said in a statement the clot was discovered during the course of a follow-up exam. There has been no announcement yet on the location of the clot.
"She is being treated with anti-coagulants and is at New York-Presbyterian Hospital so that they can monitor the medication over the next 48 hours," Reines said.
Doctors are expected to make a determination shortly about whether any further action is required, Reines said, adding that doctors will continue to assess Clinton's condition, "including other issues associated with her concussion."
Clinton was expected to return to work next week, almost three weeks after being sidelined by the concussion.
The secretary of state had been recuperating at home.
The seriousness of a blood clot "depends on where it is," said Dr. Gholam Motamedi, a neurologist at Georgetown University Medical Center who was not involved in Clinton's care.
Clots in the legs are a common risk after someone has been bedridden, as Clinton may have been for a time after her concussion. Those are "no big deal" and are treated with six months of blood thinners to allow them to dissolve on their own and to prevent further clots from forming, he said.
A clot in a lung or the brain is more serious. Lung clots, called pulmonary embolisms, can be deadly, and a clot in the brain can cause a stroke, Motamedi said.
Keeping Clinton in the hospital for a couple of days could allow doctors to perform more tests to determine why the clot formed, and to rule out a heart problem or other condition that may have led to it, he said.
Dr. Larry Goldstein, a neurologist who is director of Duke University's stroke center, said blood can pool on the surface of the brain or in other areas of the brain after a concussion, but those would not be treated with blood thinners, as Clinton's aide described.
Aides and doctors say Clinton contracted a stomach virus in early December and became dehydrated, then fainted, fell and hit her head. She was diagnosed with a concussion on Dec. 13 and hasn't been seen in public since.
Clinton was forced to cancel Dec. 20 testimony before Congress about a scathing report into the Sept. 11 attack on the U.S. diplomatic mission in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans. The report found that serious failures of leadership and management in two State Department bureaus were to blame for insufficient security at the facility. Clinton took responsibility for the incident before the report was released, but she was not blamed.
Some conservative commentators suggested Clinton was faking the seriousness of her illness and concussion to avoid testifying, although State Department officials vehemently denied that was the case.
Lawmakers at the hearings -- including Sen. John Kerry, D-Mass., the Senate Foreign Relations Committee chairman who has been nominated by President Barack Obama to succeed Clinton -- offered her their best wishes.
The former first lady and senator, who had always planned to step down as America's top diplomat in January, is known for her grueling travel schedule. She is the most traveled secretary of state in history, having visited 112 countries while in the job.
Clinton is considered a front-runner for the Democratic presidential nomination in 2016, although she has not announced plans to run.
The secretary of state's husband, former President Bill Clinton, suffered from heart disease and underwent successful quadruple bypass surgery in 2004 at Columbia Presbyterian Medical Center in New York City. In 2010, Bill Clinton had two coronary stents implanted in his heart after being hospitalized for chest pain.