Smith and Thomas told ABC News that their 33-year-old son died of a cardiac arrest caused by a seizure disorder that led to his retirement from football in June 2021.
Dr. Ann McKee, professor of neurology and pathology at Boston University’s CTE Center, examined Thomas’ brain and made a diagnosis.
“We found something we’ve seen in many other players under 34,” McKee told ABC News. “Against the background of multiple lesions in the frontal lobes and temporal lobes, the degeneration of the deeper parts of the brain begins. He was diagnosed with CTE, stage II.”
She added: “CTE by itself does not cause death. You don’t die from CTE… What CTE does is change your behavior and your personality.”
Thomas’s parents told ABC News they saw their son suffer from many of the symptoms of CTE in the months before his death, such as paranoia, memory loss and frequent headaches.
“His mood changed and he also isolated himself at times,” Smith said. “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.”
When doctors began to explain the side effects of CTE, Thomas’ father said that “the bells started ringing.”
“When they said that, I thought: “Dude, he did it … He did it too!” Bobby Thomas recalled in an interview with ABC News.
Smith said that they decided to donate Thomas’ brain for study because that’s what he would like.
“At first I didn’t want to do it. I was against it,” Smith said. “But then I remembered a conversation with DT in which he said: “Mom, if something happens to me, I want to help other players.”
Thomas is considered one of the best wide receivers in Denver Broncos history. He spent most of his career in Denver, winning two AFC championships and a Super Bowl 50. The NFL star has also been selected to multiple Pro Bowls and is second in receiving yards in Broncos history.
Thomas was traded to the Houston Texans in the 2018 season and later retired from the New York Jets.
CNN contacted the NFL regarding Thomas’s diagnosis.
Aya Elamroussi and Andy Rose of CNN contributed to this report.