Smith and Thomas told ABC News their 33-year-old son died of cardiac arrest caused by a seizure disorder that led to his retirement from football in June 2021.
dr Ann McKee, a professor of neurology and pathology at Boston University’s CTE Center, examined Thomas’s brain and made the diagnosis.
“We found what we’ve seen with so many other players under the age of 34,” McKee told ABC News. “Due to multiple lesions in the frontal and temporal lobes, degeneration of deeper brain areas begins. He was diagnosed with stage two CTE.”
She added: “CTE itself does not cause death. You don’t die from CTE. … What CTE does is it changes your behavior and your personality.”
Thomas’ parents told ABC News they saw their son struggle with many CTE symptoms, including paranoia, memory loss and frequent headaches, in the months leading up to his death.
“His mood would change and he would also isolate at times,” Smith said. “He said, ‘Mom, I don’t know what’s going on with my body. I have to pull myself together.’ And he said, ‘I don’t feel like myself anymore.’”
When doctors began explaining the side effects of CTE, Thomas’ father said, “The bells started ringing.”
“When they said it, I was like, ‘Man, he did that … He did that too!'” Bobby Thomas recalled to ABC News.
Smith said they decided to donate Thomas’s brain to studies because he would have wanted it.
“I didn’t want to at first. I was against it,” Smith said. “But then I remembered a conversation between DT and me, where he said, ‘Mom, if anything ever happens to me, I want to be able 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 the Super Bowl 50. The NFL star has also been selected for multiple Pro Bowls and is the second all-time in Broncos history for rushing yards to obtain.
Thomas was traded to the Houston Texans in the 2018 season and later retired with the New York Jets.
CNN has reached out to the NFL regarding Thomas’ diagnosis.
CNN’s Aya Elamroussi and Andy Rose contributed to this report.