Demaryius Thomas' parents said his death was caused by a seizure disorder that led to cardiac arrest.
In an exclusive interview with ABC's Good Morning America, Bobby Thomas and Katina Stuckey Smith disclosed that they donated his brain to research. Doctors discovered that the former NFL wide receiver suffered from Stage 2 CTE.
Dr. Ann McKee explained that CTE doesn't directly cause death, but "changes your behavior and your personality." Thomas' parents said he began experiencing violent seizures in 2021, which McKee believes were caused by a car crash and falling down stone stairs.
"He would shake so much, like, he couldn't breathe," his father said. "You could hear him say, 'tch-tch-tch-tch,' like the wind tryin' to come out ...They got to the point where he was having three or four back to back."
Bobby Thomas said they were told his son "suffocated" before being found unresponsive in his shower last December. He was 33 years old.
Both parents said they noticed changes in his behavior last year that aligned with CTE symptoms.
"He was paranoid, like, all the time. But memory loss, I saw that, as well," Bobby Thomas recalled. "Every single day, he complained about -- about having a headache."
"His mood would change, and he would also isolate himself sometimes," Smith added. "[Demaryius] would tell me, he was like, 'Mom, I don't know what's goin' on with my body -- I gotta get myself together.' And he said, 'I don't feel like myself anymore.'"
Demaryius Thomas retired from the NFL in June. His father said the former Denver Broncos star sought medical assistance and tried to better understand what he was experiencing in the months leading up to his death.