Queen Elizabeth II is said to be in "good spirits" after spending a night in hospital.
Buckingham palace confirmed the 95-year-old monarch underwent "preliminary investigations" on Wednesday afternoon (UK Time) and returned to Windsor Castle the next day.
The Queen had cancelled a scheduled trip to Northern Ireland earlier in the week.
"Following medical advice to rest for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today [Thursday], and remains in good spirits," the palace said in a statement.
After hosting a drinks reception on Tuesday night at Windsor Castle for billionaire business leaders like Bill Gates, Buckingham Palace announced the Queen would be taking a break, on doctor's orders.
"The queen has reluctantly accepted medical advice to rest for the next few days," the palace said this week.
"Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland, where she had been due to undertake a series of engagements today and tomorrow."
According to The Sun, the monarch only stayed overnight at the hospital due to it being too late to get back home, and she was reportedly back at her desk undertaking light duties the following day.
Concerns about the Queen's health were first raised when she began using a walking stick while attending engagements at Westminster Abbey and the Welsh parliament this month.