Georgia Tech football star Demaryius Thomas, who died in December at the age of 33 of an apparent seizure, was found to have been suffering from the degenerative brain disease known as chronic traumatic encephalopathy, or CTE.

The announcement was made Tuesday by the Boston University CTE Center, which had received Thomas’ brain from his family. Thomas was diagnosed with CTE stage 2. According to the release, symptoms of a stage-2 diagnosis include impulsivity, depression, executive dysfunction and memory loss. The release also said that Thomas had suffered from daily headaches, chronic pain and vision issues in his 20s, and in his 30s had experienced anxiety, depression, apathy and worsening memory loss.

CTE, which can be diagnosed only posthumously, has been widely linked with the playing of football. In 2019, Boston University researchers found that, because of the game’s pounding and repeated head collisions, “there is a strong relationship between CTE risk and the number of years a person plays football.” At the professional, college and high school levels, concussion protocols have been instituted to protect players.

Thomas’ parents told ABC News that he had been demonstrating erratic behavior.

“His mood would change, and he would also isolate himself sometimes,” Thomas’ mother, Katina Smith, said. “(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.’”

Bobby Thomas, Thomas’ father, also witnessed those behavior changes.

“He was paranoid, like, all the time. But memory loss, I saw that, as well,” Thomas said. “Every single day, he complained about, about having a headache.”

Thomas did not die from CTE, however. Boston University neuropathologist and CTE Center director Ann McKee told ABC that CTE does not cause death, but that “what CTE does is it changes your behavior and your personality.”

The Fulton County Medical Examiner’s Office had yet to release a public report on the cause of Thomas’ death as of Tuesday, but the family said that Thomas died from a seizure, which is not a common symptom of CTE. According to his family, Thomas had begun suffering seizures since a 2019 car accident. He retired from football in June 2021.

“He was paranoid, like, all the time. But memory loss, I saw that, as well. Every single day, he complained about, about having a headache."

- Bobby Thomas, Demaryius Thomas’ father

It was a tragic final chapter for a young man who had overcome the most trying circumstances in his youth growing up in Laurens County in Middle Georgia to achieve stardom on the football field and bring joy to family, friends and fans with a giving nature and an irrepressible smile.

After achieving All-American status at Tech, Thomas became a first-round draft pick of the Denver Broncos, the start of a 10-year NFL career in which he earned five Pro Bowl designations and won a Super Bowl.

Smith said that, after her son’s death, she was against donating his brain for CTE research.

“And then I remember a conversation DT and I had where he said that, you know, ‘Mom, if anything ever happens to me, I wanna be able to help other players,’” she said.