In an interview with George Stephanopoulos on Good Morning America, the 39-year-old actor was asked if he had any hesitation about taking the genetic test that ultimately revealed that his risk of developing Alzheimer's is eight to 10 times higher than that of the general population.

"To be honest, I didn’t know what the test was at the time," Hemsworth told Stephanopoulos. "The idea was that we were gonna do a deep dive into my blood work, look at my genetics and on camera tell me what they found. And I didn’t expect to find much."

To the contrary, longevity specialist Dr. Peter Attia informs Hemsworth during a preview of the brain health episode that he has two copies of APOE4 (the apolipoprotein E gene), one from his mom and one from his dad. 

"And what does that mean, exactly?" Hemsworth asks during the episode, to which Attia answers, "That means you have an increased risk of Alzheimer's disease."

Hemsworth then asks, "How much higher?" and Attia replies, "Probably somewhere between eight and 10 times higher than that of someone in the general population."

Hemsworth, during a confessional in the series, reveals that "the idea that I won't be able to remember the life I've experienced, or my wife, my kids, is probably my biggest fear."

The Thor star said on GMA that he first learned of the genetic test results off-camera, saying the doctor told him it's not something he wanted him to first find out with cameras rolling. Hemsworth said the doctor wanted to give him the opportunity to accept or decline revealing the results on the show.

Despite the news, Hemsworth agreed with the doctor that the discovery of the APOE4 gene is a "blessing," and he explained why.

"It very quickly just became another wonderful motivation to make some changes and arm myself with all the tools to live a healthier, better life," he said. 

Earlier this month, ET had an exclusive sneak peek at Hemsworth attempting a major stunt as part of a six-part exploration of the human body's ultimate potential. In that preview, Hemsworth struggles to calm his nerves as he scales a skyscraper in preparation for an unnamed but seemingly very dangerous adventure. 

From below, a woman monitors Hemsworth's heart rate and ventilation while he reaches the summit of the building.

"My feet feel incredibly heavy," he says just before the adventure begins. "Just trying to enjoy the view and be comfortable in a very uncomfortable situation." 

Hemsworth said each episode of Limitless, streaming now on Disney+, concentrates on a different pillar of health, wellness and longevity, where he tests the science by putting himself through a series of different extreme physical and emotional challenges.

From swimming in freezing waters to climbing a rope 1,000 feet off the ground, Hemsworth revealed that a four-day, water-only fast proved to be the most challenging of them all.

Why? He got hangry?