Queen Elizabeth was hospitalized Wednesday night for "preliminary investigations," a Buckingham Palace spokesman confirmed to ABC News.
The queen was back at her desk at Windsor Castle by Thursday afternoon and undertaking light duties.
No other details about the queen's condition are currently available.
"Following medical advice to rest for a few days, The Queen attended hospital on Wednesday afternoon for some preliminary investigations," the palace said in a statement. "[She] remains in good spirits."
Queen Elizabeth, 95, hosted a reception for leaders, including Bill Gates and John Kerry, at Windsor Castle on Tuesday.
The next day she was forced to cancel a trip to Northern Island as her medical team advised her to get some rest.