Baroness Thatcher is expected to stay in hospital until at least the start of next week while being treated for an infection, her aides say.
Sir Mark Thatcher said his mother Margaret was "chirpy" and had been talking about the Spending Review.
It is understood the ex-prime minister has not fully recovered from an illness which prevented her attending Downing St last week to mark her 85th birthday.
A spokesman added: "We expect her to be staying in hospital into next week."
Prime Minister David Cameron wished Baroness Thatcher a "speedy recovery".
And her former aide, the Conservative MP Gerald Howarth, said: "We all wish her well, and are sorry we did not see her last week."
Baroness Thatcher, who was prime minister from 1979 to 1990, was taken to Cromwell Hospital, west London, on Tuesday evening in what has been described as a "routine admission".
The BBC understands that following a bout of flu, she has been suffering from a condition affecting her muscles, which has made it difficult for her to move around.
Sources say the condition is "eminently treatable".
Sir Mark Thatcher spent an hour at the hospital on Wednesday morning and said his mother was doing well but was "drowsy".
Speaking after his visit, Sir Mark said she was likely to remain in the private Cromwell Hospital for several days.
He said: "They have started the tests and when they get the results they will make the appropriate decisions. She's a little bit drowsy but she had a good night and is slightly sedated at the moment."
In 2005, the former PM was advised by doctors that she should not make public speeches in the wake of some minor strokes.
But she still attends some public functions, including a recent address by the Pope during his state visit to the UK.
In March 2008, she was taken ill during a dinner in Westminster and spent the night in hospital as a precaution, but was later given a clean bill of health.
Rupert Murdoch - the businessman who owns a range of media interests including The Sun, Times and Sky TV in the UK - is expected to deliver the inaugural Margaret Thatcher lecture as planned later on Thursday.
His address, entitled Free Markets and Free Minds, is expected to praise Lady Thatcher's contribution to Britain's economy and society.