The US Secretary of State collapsed and hit her head after becoming dehydrated while suffering from a stomach virus in mid-December.
She was expected to return to work on Monday but a spokesman said that a test on Sunday had revealed a blood clot.
The 65-year-old was taken to New York-Presbyterian Hospital in Manhattan and is being treated with anti-coagulants.
She is expected to stay in hospital for 48 hours while doctors monitor the effect of the medication.
I wouldn’t say that I am an alcoholic. I don’t wake up yearning for a drink, like a man I knew 20 years ago. He worked for the Evening Standard and used to bring a flask of orange juice, laced with vodka, with him into the office before first edition (which went to bed at 8.45am).
He is now dead and I think of him a lot and miss him very much. While he was alive we often used to discuss drink, a subject of keen mutual interest. He held that the ideal alcoholic state was what he called “mellow”, where the drinker felt a sense of warmth and generosity towards the world.
When my friend was in this state, he would empty his pockets and try to force £20 notes on his fellow drinkers. Later, he went bankrupt. I have an acquaintance – who is, I think, still alive – who used to hide bottles of whisky all around his house, in strategic places like his sock drawer. He suffered from terrible boils, which would break out without warning in embarrassing and painful places.
In an article for The Telegraph, Iain Duncan Smith accuses Labour of establishing a system of tax credits for the lower-paid that is “wide open to abuse”.
HM Revenue and Customs conducts just 34,000 checks a year on tax credit claims deemed to be “high risk” – less than a tenth of the number of investigations into benefits fraudsters, he writes. As a result public finances have been pushed to “breaking point”.
Mr Duncan Smith’s attack on tax credits represents a new assault by ministers on Britain’s welfare system. Previously, ministers have focused on the abuse of benefits but George Osborne has insisted that further savings need to be found to help repair the public finances.
Tax credits were introduced by Gordon Brown to top up the incomes of the lower-paid, particularly those with children, and are regarded by Labour as one of the party’s proudest achievements. They are overseen by the Treasury rather than Mr Duncan Smith’s department.
A low-paid family with two children in child care can receive more than £10,000 annually from the tax credit system. But official figures show that about one in 12 tax credit claims is incorrect or fraudulent – compared with fewer than one in 25 benefit claims.