The US secretary of state, Hillary Clinton, has been admitted to hospital after doctors found a blood clot related to a concussion she suffered earlier this month.
Doctors made the discovery during a follow-up exam at New York-Presbyterian hospital, Philippe Reines, the deputy assistant secretary of state, told reporters.
"Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago.
"Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required."
Clinton was expected to stay in hospital for 48 hours while specialists monitor her condition and treat her with anti-coagulants, Reines added.
Clinton, 65, had a stomach virus when she fainted due to dehydration three weeks ago, causing the concussion. She had been expected to return to work this week after recuperating at home.
She missed a planned trip to the Middle East and north Africa as well as congressional hearings into the September attack on the American consulate in Benghazi, Libya, which killed four Americans including the ambassador, Chris Stevens.
Clinton is stepping down as secretary of state to rest and, it has been speculated, to pave the way for a White House run in 2016. The sudden health concerns have placed her age in the spotlight and a question mark over any presidential bid.
The former first lady has been noted as a particularly hard-working diplomat. Earlier this year, she alluded to the effects of heavy travelling. Asked in January at a state department staff event of her intentions after the presidential election, she replied: "I think after 20 years, and it will be 20 years, of being on the high wire of American politics and all of the challenges that come with that, it would probably be a good idea to just find out how tired I am."
She did not completely rule out a return to politics, adding: "Everyone always says that when they leave these jobs." Senior Republicans such as Newt Gingrich have said Clinton, who narrowly lost the 2008 Democratic nomination to Barack Obama, would be a formidable opponent in 2016.
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