Thatcher, 87, underwent a minor surgery Thursday to have the growth removed, the BBC reported.
Although Thatcher's spokeswoman said the former prime minister is doing "absolutely fine," her friends said she will be in the hospital for a few days.
"The operation was a complete success and her condition is satisfactory. She'll stay in hospital for a few more days and then she'll go home," said her friend and former adviser Tim Bell.
"She's very tough. The doctors are perfectly happy. They say the operation has been completely satisfactory," he said.
Thatcher, who was prime minister from 1979-90, has been in poor health in recent years, suffering from a series of mini-strokes in 2002, the BBC said.
Her daughter, Carol Thatcher, has said that her mother is also struggling with dementia.