After the cancellation of official duties, the Queen spent the night in the hospital for tests.
Royal doctors sent the 95-year-old sovereign to be seen by specialists in London on Wednesday afternoon.
Her Majesty has been discharged – Buckingham Palace said she was in “good spirits” following a “preliminary investigation” during her visit to the hospital.
His medical team was believed to have taken precautions to get him admitted to the King Edward VII Hospital in Marylebone on Wednesday.
And the overnight stay – her first in the hospital for eight years – was for practical reasons.
Yesterday afternoon, Har Major, 95, was deemed to be back at his desk at Windsor Castle.
She is said to have been reading her official government red boxes and is expected to recover for the rest of the week.
The Palace said: “After a few days of medical advice, the Queen attended hospital for some preliminary check-ups on Wednesday afternoon, returned to Windsor Castle at lunch today (Thursday) and remains in good spirits. “
Fifth major hospital visit in 40 years
It was believed that the queen stayed the night in the hospital because it was taking too long to take her home on the 26-mile journey.
She hasn’t been thought to stay in the hospital overnight since March 2013, when she was treated for a stomach bug at the same hospital.
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And it was believed to be only his fifth major hospital visit in 40 years.
Allies are expected to join world leaders at the COP26 climate change summit in Glasgow on 1 November.
On Wednesday, Her Majesty canceled a two-day visit to Northern Ireland on the advice of doctors.
Concerns were raised for the first time this month at Westminster Abbey and the Welsh Parliament introducing the use of a walking stick.
She stood on her feet for more than an hour at a business summit at Windsor Castle on Tuesday.
She has also decided to give up alcohol since the death of 99-year-old Prince Philip in April.
“After medical advice for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary check-ups, returned to Windsor Castle at lunch on Thursday and remained in good spirits.”