Mark Thatcher said his mother was "surprising good" and was hospitalized for some tests after a flu infection hadn't cleared, Sky News reported.
"She is in very good spirits, absolutely excellent," he said, adding that the tests were "entirely precautionary."
Thatcher skipped her 85th birthday party at Downing Street last week because she was ill.
"She's been recovering quite well but the doctors wanted to bring her into hospital for some routine tests over the next couple of days," Mark Thatcher said. "If they go according to plan, she'll be home shortly."