Secretary of State Hillary Clinton was admitted to a New York hospital after the discovery of a blood clot stemming from the concussion she sustained earlier this month.
Mrs Clinton's doctors discovered the clot while performing a follow-up exam, her spokesman, Philippe Reines, said.
He would not elaborate on the location of the clot but said Mrs Clinton is being treated with anti-coagulants and would remain at New York-Presbyterian Hospital for at least the next 48 hours so doctors can monitor the medication.
"Her doctors will continue to assess her condition, including other issues associated with her concussion," Mr Reines said in a statement.
"They will determine if any further action is required."
Mrs Clinton, 65, fell and suffered a concussion while at home alone in mid-December as she recovered from a stomach virus that left her severely dehydrated.
The concussion was diagnosed December 13 and Mrs Clinton was forced to cancel a trip to North Africa and the Middle East that had been planned for the next week.
The seriousness of a blood clot "depends on where it is", said Dr Gholam Motamedi, a neurologist at Georgetown University Medical Centre who was not involved in Mrs Clinton's care.
Clots in the legs are a common risk after someone has been bedridden, as Mrs 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, Dr Motamedi said.
Keeping Mrs 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 centre, 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 Mrs Clinton's aide described.
Mrs Clinton was forced to cancel December 20 testimony before Congress about a scathing report into the September 11 attack on the US 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 bureaux were to blame for insufficient security at the facility.
Mrs Clinton took responsibility for the incident before the report was released, but she was not blamed.
Some conservative commentators suggested Mrs Clinton was faking the seriousness of her illness and concussion to avoid testifying, although State Department officials vehemently denied that was the case.
Politicians at the hearings - including Democratic Senator John Kerry, the Senate Foreign Relations Committee chairman who has been nominated by President Barack Obama to succeed Mrs Clinton - offered her their best wishes.
Last Thursday, before the discovery of the blood clot, Mr Reines said Mrs Clinton was expected to return to work this week.
The former first lady and senator, who had always planned to step down as America's top diplomat in January, is known for her gruelling travel schedule.
She is the most travelled secretary of state in history, having visited 112 countries while in the job.
Mrs Clinton is considered a front-runner for the Democratic presidential nomination in 2016, although she has not announced plans to run.