Secretary of State Hillary Clinton was hospitalized on Sunday after doctors found a blood clot.
Clinton is receiving treatment at Presbyterian Hospital in New York where doctors will monitor her for the next 48 hours, her spokesman Philippe Reines said in a statement.
The clot, resulting from a concussion she suffered earlier in December, was found during a follow-up exam on Sunday, according to the State Department.
"Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required," Reines said.
The illness forced the secretary to cancel a trip to the Middle East and an appearance before Congress.
Clinton was due to testify before Congress on Dec. 20 about the investigation into the September attack on the U.S. Consulate in Benghazi, Libya, which killed four Americans.
Two senior aides to Clinton testified on her behalf, but Republicans, who have been sharply critical of the administration’s handling of security for the Benghazi mission, have said they still expect Clinton testify in person.
Clinton has said she will step down from her post as the nation’s top diplomat, and President Obama has nominated Sen. John Kerry (D-Mass.) to succeed her.
— Amie Parnes contributed to this report.
— Updated at 10:03 p.m.