Prime Minister Boris Johnson passed his best wishes on to the monarch, 95. She will spend Friday resting and undertaking light duties after spending a night in hospital for what Buckingham Palace called “preliminary investigations".
The palace did not immediately release details of her hospital admission on the grounds of privacy.
She was admitted to the private King Edward VII’s Hospital in central London and was back at Windsor Castle by lunchtime on Thursday.
The queen underwent tests after she cancelled a scheduled trip to mark 100 years since the creation of Northern Ireland.
“She hates people making a fuss of her in general but particularly to do with health,” royal expert Robert Hardman said.
“And I think there’s a concern to maintain the dignity of the office, and I know that one reason why nothing was said about yesterday’s trip to hospital was that they didn’t suddenly want huge banks of cameras and 24-hour news setting up outside the hospital.”
The queen was seen using a walking stick at a Westminster Abbey service marking the centenary of the Royal British Legion, an armed forces' charity.
In recent days, she has also travelled to Cardiff in Wales, hosted a major global investment summit at Windsor Castle with world leaders and enjoyed a day watching horseracing at Ascot.
Next month she is scheduled to attend the Cop26 summit in Glasgow.
The queen’s royal physicians are understood to be taking a “cautious approach”.
During the coronavirus pandemic, the queen retreated to Windsor Castle for her safety, where she was joined by the Duke of Edinburgh in lockdown.
Britain’s longest-lived and longest-reigning monarch has enjoyed robust health throughout her life.
She is scheduled to celebrate her Platinum Jubilee, 70 years on the throne, next year.
She was widowed this year when Prince Philip died at age 99 in April. She has cut back on her workload in recent years but still keeps a busy schedule of royal duties.