"In the course of a follow-up exam today, Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago," her spokesperson and the Deputy US Assistant Secretary of State, Philippe Reines, said in a statement.
"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.
"Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required," he added.
Clinton (65), who suffered a stomach infection early this month, and was working from home for most part of the December, was expected to return to work next week.