Last week, she used a walking stick during an engagement for "comfort" for the first time, and it emerged she has given up drinking alcohol to stay in top shape after taking doctors' advice.
Royal commentators raised questions over how the matter had been dealt with, urging Buckingham Palace to act with more transparency on the deeply serious matter of the Queen's health.
"The problem, it seems to me, is that rumour and misinformation always thrives in the absence of proper, accurate and trustworthy information," Nicholas Witchell, the BBC's royal correspondent, said.
"We're told that she's in good spirits. That would certainly be in line with her stoical character, but that's a phrase that's a little bit of a cliche now.
"We're told that she is back at Windsor Castle undertaking light duties. Well, we must hope we can place reliance on what the palace is telling us."
Late on Thursday night, in response to a story in The Sun, the palace released a statement to say: "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, and remains in good spirits."
The journey from Windsor to the King Edward VII's hospital in London was made by car.
Unlike more routine matters, when the Queen's medics are able to examine and treat her at home, the tests required specialist doctors and medical equipment only available in hospital.
The overnight stay was for "practical" reasons, a source said.
It is understood that the Queen was due to stay in hospital only for a short period, with aides choosing not to make a public statement in order to protect her privacy.
The Royal Standard remained flying at Windsor Castle throughout the overnight trip.
The Queen is now understood to have conceded to work "light duties" only, dealing with her red box, reading paperwork, making some calls and seeing close aides, such as her Private Secretary and Diary Secretary.
The palace is awaiting further information before confirming whether she will be taking part in any public engagements next week.
The Queen is scheduled to travel to Glasgow to attend Cop26 on November 1, with supporters urging her not to feel any pressure to appear at the climate change summit.
A Downing Street spokesman declined to say whether Boris Johnson had held his weekly audience with the monarch, but so far it has not been listed in the Court Circular.
The spokesman said: "The Prime Minister's best wishes have been passed on to Her Majesty."
Joe Little, the managing editor of Majesty magazine, said the issue of revealing details about the monarch's health is problematic.
"This is a tricky one because the Queen does have a right to a certain degree of privacy," he said. "But on the other hand, she's head of state. So does that entitle us to know exactly what ailments she may or may not have? It's a very difficult one to get the balance right for the satisfaction of everybody."
The Queen's admittance was her first overnight stay in hospital since she spent a night at the private clinic in 2013 when she was treated for gastroenteritis symptoms.
