British Prime Minister David Cameron has wished Margaret Thatcher a "speedy recovery" via a message on Twitter.
Lady Thatcher, 87, is in hospital after a minor operation. She was admitted on Thursday.
Friends of the former prime minister told the BBC they expect her to remain in hospital for a number of days.
Baroness Thatcher served three terms between 1979 - 1990. She had a series of minor strokes in 2002 and was advised by doctors to stop making public speeches.
Her daughter, Carol Thatcher, has previously spoken of her mother's struggle with dementia.
The BBC reports the former prime minister was not well enough to join the Queen for a lunch with the prime minister and former prime ministers as part of the Diamond Jubilee celebrations this year.
And two years ago she was unable to attend an 85th birthday party for her hosted by Mr Cameron at 10 Downing Street.
However, she was sufficiently well in October to mark her 87th birthday with lunch at a London restaurant with her son Mark and his wife.