Buckingham Palace said the Duke would receive tests for a pre-existing heart condition as well as receive treatment for an infection.
"And the jab – it didn’t hurt at all," Queen Elizabeth said in remarks released by Buckingham Palace.
Prince William, son of heir Prince Charles, said on Monday his grandfather was "OK, they're keeping an eye on him."
A royal source said doctors had been acting out of an abundance of caution and that Philip was in good spirits.
Charles arrived at the back of the London hospital where Philip, the 99-year-old husband of Queen Elizabeth, has spent four nights.
There were no reports of visitors arriving at the private hospital on Thursday.
A royal source stressed that the admission was not an emergency.
The Duke of Edinburgh was taken to hospital on Tuesday, Buckingham Palace said.
Queen Elizabeth is 94-years-old, while Prince Philip is 99.