Hillary Clinton, seen here in file footage, was admitted to a New York hospital in Sunday with a blood clot.
The U.S Secretary of State is said to have developed the clot after a concussion when she fainted earlier in December.
She's being treated with anti-coagulants and was due to stay in hospital for 48 hours.
Clinton earlier this month cancelled an overseas trip because of a stomach virus.
Officials subsequently said her condition was improving and have played down suggestions it was anything more serious.
Clinton treated for blood clot (0:35)
Dec.31 - U.S. Secretary of State Hillary Clinton is undergoing hospital treatment for a blood clot. Paul Chapman reports. ( Transcript )