The Chappaqua resident was undergoing a follow-up exam related to a concussion she suffered on Dec. 13, her spokesperson said, when her doctors found the clot.
The Secretary of State is being treated with anticoagulants, according to Philippe Reines, deputy assistant secretary of state, and will remain in the hospital for the next 48 hours.
Clinton was scheduled to return to work this week afte a three-week absence, CNN reported.
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