- He left Clarence House, London, for his Norfolk residence on Wednesday, accompanied by the Queen.
- His younger son, the Duke of Sussex, had flown in from the US for a brief visit earlier that day.
King Charles has given his "heartfelt thanks" for messages of support from the public, in his first statement since being diagnosed with cancer.
The 75-year-old monarch said: "As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement."
News of the diagnosis was announced by Buckingham Palace on Monday.
The King is being treated for an unspecified form of cancer and is currently staying at Sandringham.
His cancer was detected while he was undergoing treatment for an enlarged prostate in January. While the type of cancer has not been disclosed, the Palace confirmed it was not prostate cancer.
In the message of thanks, he wrote: "It is equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organisations which support cancer patients and their families across the UK and wider world.
"My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience."
King's cancer treatment
The King has stepped back from all public-facing duties while he is being treated for the cancer, with senior royals - including Queen Camilla and Prince of Wales - taking on his duties for some events.
He left Clarence House, London, for his Norfolk residence on Wednesday, accompanied by the Queen. His younger son, the Duke of Sussex, had flown in from the US for a brief visit earlier that day.
"I have to be seen to be believed" was one of the guiding principle of the late Queen Elizabeth II. Well, the King won't be seen any time soon, the whirlwind of engagements is paused for the moment.
But he will be heard from.
And by the normally very restrained standards of Royal prose, this is pretty personal.
Not many people of his age have chosen to make their medical condition a global talking point. And the King goes out of his way to express his hope that his relative openness has served a purpose.
And there's more - this is a man who, like so many cancer patients, is now in hands of the doctors and the nurses. And like many patients he goes out of his way to highlight the work of those who care, work he has of course come into contact with before, but never like this.
The thanks the King gives for all the messages of support is unsurprising royal fare. But even there, the meeting of man and monarch in diagnosis and treatment is revealed - those messages, he says in rather different words, mean a lot.
The Queen had said her husband was "very touched by all the letters and messages the public have been sending from everywhere".
Speaking at an event at Salisbury Cathedral on Thursday, she said the support had been "very cheering".
Prince William had also expressed gratitude for messages of support for his father and also for his wife, the Princess of Wales, as she recovers from abdominal surgery.
She is not expected to return to royal duties until after Easter.
Speaking at a charity fundraiser for London's Air Ambulance, attended by guests including Hollywood star Tom Cruise, Prince William said the comments meant "a great deal to us all".
"It's fair to say the past few weeks have had rather a 'medical' focus," he said.
The King's sister, the Princess Royal, acknowledged words of support for both the monarch and Catherine earlier this week. Anne attended the Six Nations match between Scotland and France on Saturday, in her role as the patron of Scottish Rugby