US Secretary of State Hillary Clinton is said to be "in good spirits" despite the blood clot which has been identified in a vein between her brain and skull exactly behind her right ear.
The physicians who are treating the 65-year-old have also ruled out that she suffered a stroke or a neurological breakdown.
"She is in good spirits, engaging with her doctors, her family and her staff," said the doctors in a statement released by the State Department, adding that Clinton is making "excellent progress". She is likely to make full recovery quite soon.
Clinton has been admitted to the New York Presbyterian Hospital with a blood clot stemming from a concussion she suffered earlier in December.
The concussion, sustained when Clinton fainted and fell on 9 December, has left the former first lady incapacitated. She has not been seen in public since being diagnosed with the concussion on 13 December.
"In the course of a routine follow-up MRI on Sunday, the scan revealed that a right transverse sinus venous thrombosis had formed. This is a clot in the vein that is situated in the space between the brain and the skull behind the right ear," said doctors Lisa Bardack and Gigi el-Bayoumi.
They added: "To help dissolve this clot, her medical team began treating the secretary with blood thinners. She will be released once the medication dose has been established."
Earlier, State Department spokesperson Philippe Reines said Clinton had been treated with anti-coagulants at the Presbyterian Hospital.
US officials have been playing down the concussion since it was diagnosed, despite constant press speculation. During the four years of her term, Clinton has flown over a million miles and travelled to more than a hundred countries across the world.
Her term is due to end within weeks as she has not sought re-election to President Barack Obama's administration which begins its second term in January 2013.
Clinton missed her scheduled testimony before the Congressional committee investigating the attack on the US embassy in Benghazi, Libya which killed four people including ambassador Chris Stevens. However, she has insisted she will appear before the committee before relinquishing office.
Clinton, who cancelled her appearance on 15 December, has publicly accepted responsibility for the security failures which preceded the Benghazi attack.