Baroness Thatcher is expected to stay in hospital into next week, a spokesman has said.
The former prime minister was admitted to the private Cromwell Hospital in London for tests on Tuesday night after failing to recover from flu.
An aide said he had no update on her condition, but added that she would be remaining in hospital for the next few days.
Lady Thatcher's son Mark said she was "in good order" and had been talking about the Government's Comprehensive Spending Review (CSR).
He said: "She's a little bit drowsy but she had a good night and is slightly sedated at the moment."
He added: "She's good, chirpy, talking about the CSR, so in good order."
Doctors decided that she needed to be admitted for assessment.
Prime Minister David Cameron took time out from preparations for the CSR to wish Lady Thatcher a "speedy recovery".
Last week the Tory peer was forced to withdraw from a Downing Street reception to mark her 85th birthday after coming down with flu.
Friends said she was still being troubled by the illness on Tuesday afternoon, and aides called in a doctor to assess her condition.