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An autopsy on former Denver Broncos receiver Demaryius Thomas revealed that he died of complications from a seizure disorder.

Thomas was 33 when he died on December 9, 2021, just two weeks before his birthday.

The 10-year NFL veteran was diagnosed with stage 2 chronic traumatic encephalopathy, a degenerative brain disease, at the time of his death. However, the Fulton County Medical Examiner saw no connection between the two diagnoses.

CTE does not directly cause death.

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An autopsy also showed traces of nicotine and marijuana in his system, but neither was said to have played a role in his death.

After a car accident in 2019, Thomas began having seizures and friends and family noticed a change in his behavior.

Demaryius Thomas of the Denver Broncos celebrates a touchdown during the 2015 AFC Divisional Playoff game against the Indianapolis Colts at Sports Authority Field at Mile High Jan. 11, 2015, in Denver, Colo.

Demaryius Thomas of the Denver Broncos celebrates a touchdown during the 2015 AFC Divisional Playoff game against the Indianapolis Colts at Sports Authority Field at Mile High Jan. 11, 2015, in Denver, Colo.
(Harry Howe/Getty Images)

Former NFL star Demaryius Thomas has stage 2 CTE, family says

“His mood swings and he even isolates himself sometimes,” his mother, Katina Stuckey Smith, told ABC News. “Demaryius used to tell me, he’s like, ‘Mom, I don’t know what’s going on with my body. I have to get myself together.’ And he said, ‘I don’t feel like myself anymore.’

Thomas was a four-time Pro Bowler, playing in two Super Bowls and winning one. He put up at least 1,000 receiving yards in five straight seasons from 2012-2016.

A tribute logo to the late former Denver Broncos receiver Demaryius Thomas at Mile High at Empower Field on Dec. 12, 2021 in Denver, Colo.

A tribute logo to the late former Denver Broncos receiver Demaryius Thomas at Mile High at Empower Field on Dec. 12, 2021 in Denver, Colo.
(Justin Edmonds/Getty Images)

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He played 8½ seasons with the Broncos before ending the 2018 season with the Houston Texans. He played for the New York Jets in 2019. He signed with the New England Patriots but never played in a regular season game for them.