Jeremy Paxman's doctor diagnosed him with Parkinson's disease after noticing that he was less "exuberant" on University Challenge, the veteran broadcaster has revealed.
The former Newsnight presenter said the diagnosis came when he was in hospital after collapsing while walking his dog.
One of the doctors said he had noticed that the 72-year-old's face had acquired a "Parkinson mask".
"Well, it was completely out of the blue," Paxman said of the diagnosis.
"I was having a walk in the square across the way. There was ice around and I had the dog with me - the dog was on a lead.
"The first thing I knew was when somebody was sitting me on a bench. I'd fallen over and I made a terrible mess of my face.
"I'd gone straight down on my hooter, which, as you can see, is not small. Cuts all over the place. I was a real mess. And when I was in A&E, a doctor walked in and said, 'I think you've got Parkinson's'.
"And it turned out that he had been watching University Challenge and had noticed that my face had acquired what's known as the 'Parkinson mask'.
"I wasn't as effusive and exuberant as normal. I had no idea."
Parkinson's disease is a condition in which parts of the brain become progressively damaged over many years.
The three main symptoms of the condition are involuntary shaking of particular parts of the body, known as tremors, slow movement, and stiff and inflexible muscles.
He said he decided to stop, as it will become obvious to viewers that there is something "unusual" about him.
"I think I've been doing University Challenge for about 29 years, which is a very, very long time - too long really," he said.
"I always think that if you're scared of making a decision, do it."
"The University Challenge people have been brilliant, but I do think that I ought to stop doing it. I will be sad to give it up, but no one is indispensable," he added.
"It'll become obvious that there's something funny about me - and I mean unusual, not funny-funny. So I don't want to spoil it for them."
However, Paxman will not be leaving screens just yet.
He presents a one-off special on ITV about his diagnosis - Paxman: Putting Up with Parkinson's - at 9:00pm on ITV tonight, Tuesday 4 October.
If you have been affected by issues raised in this story, please visit: www.rte.ie/helplines.
Source: Press Association
