The Marvel star discovered a shocking health risk while filming his new TV series for National Geographic.

Chris Hemsworth discovered he has a high risk of developing Alzheimer’s disease while filming his new docuseries.

'Limitless' for National Geographic, which can be streamed on Disney+, follows the 39-year-old action movie star as he pushes himself to his absolute physical and mental limits by doing incredibly difficult challenges that test his mind and his body. The series' fifth episode premiered recently, which saw him undergo a series of genetic tests. These, unfortunately, confirmed Hemsworth's "biggest fear", and resulted in him wanting to take time off from work to spend more time with his family.

In an interview with Vanity Fair, Chris Hemsworth opened up about his higher risk of potentially having Alzheimer’s disease as he ages. The Australian star, known for playing Thor in Marvel's Cinematic Universe, learned he had two copies of the gene ApoE4 which he has inherited from both his parents. This means he is among 2-3% of the population who are up to 10 times more likely to develop the disease than those without.

He told the publication: "It’s not like I’ve been handed my resignation" but the news did trigger "something in me to want to take some time off".

Hemsworth was initially going to be given the test results live on camera, but due to the serious nature of the outcome, 'Limitless' creator Darren Aronofsky decided to tell him in private. Hemsworth was then given the option of deleting all references to Alzheimer’s on the show, but the star thought it was better for the audience to hear about his story.

"My concern was I just didn’t want to manipulate it and overdramatise it, and make it into some sort of hokey grab at empathy or whatever for entertainment," he told the publication.

"If you look at Alzheimer’s prevention, the benefit of preventative steps is that it affects the rest of your life. It’s all about sleep management, stress management, nutrition, movement, fitness. It’s all kind of the same tools that need to be applied in a consistent way."

Hemsworth's grandfather is currently being treated for Alzheimer’s.

While it's not a definite diagnosis, the movie star did say it was "a strong indication" of what may come for him as he gets older. For now, however, he will be staying at home with his wife Elsa Pataky and their three children as he takes a break from acting: "I’m going to have a good chunk of time off and just simplify. Be with the kids, be with my wife."

Watch 'Limitless' on Disney+ now.