Secretary of State Hillary Clinton is making “excellent progress” after a blood clot was found between her brain and skull, doctors say.
Doctors said Hillary Clinton will make a full recovery and will be released from hospital once a blood-thinning dosage has been set.
Hillary Clinton, 65, was admitted to a New York hospital on Sunday, having suffered a concussion earlier in the month when she fainted and fell over.
She is due to stand down before President Barack Obama begins his second term.
Doctors said an MRI scan on Sunday had revealed the formation of a “right transverse sinus venous thrombosis” – a clot in the vein behind Hillary Clinton’s right ear in the space between the brain and the skull.
“It did not result in a stroke, or neurological damage,” said Dr. Lisa Bardack and Dr. Gigi El-Bayoumi, who were treating Hillary Clinton at New York’s Presbyterian hospital.
The secretary of state was said to be in good spirits and “engaging with her doctors, her family, and her staff,” added the doctors in a statement.
Doctors discovered the clot during a routine follow-up MRI scan on Sunday, said her spokesman, Philippe Reines, adding that she was being treated with anti-coagulants.
At the time of her faint, Hillary Clinton was reported to have had a stomach virus and to have passed out after becoming dehydrated.
Hillary Clinton is due to give evidence before a Congressional committee in January in connection with the attack in September on the US consulate in the Libyan city of Benghazi.
The US ambassador to Libya and three American officials were killed in the incident.
Hillary Clinton was appointed secretary of state at the start of Barack Obama’s first term, in January 2009.
Her most recent foreign trip was to Dublin earlier in December: she is renowned for keeping a gruelling travel schedule.
She is the most travelled secretary of state in history, having visited 112 countries while in the job, the Associated Press says.
Earlier this month, President Barack Obama nominated Senator John Kerry – the Massachusetts Democrat who heads the Senate Foreign Relations Committee – to replace Hillary Clinton as secretary of state.
Hillary Clinton has repeatedly said she only intended to serve one term in the post.