The former tight end penned a retirement letter on The Players' Tribune detailing the reason he's walking away from football.
"I'm ready to admit that I'm OK with giving up the identity of 'Julius the football player.' I'm O.K. with not being recognized as an NFL athlete -- because I'm more than that," Thomas wrote. "I have stepped out of my identity before, and I will continue to do it again and again throughout life. I believe helping others is what I'm supposed to be doing at this point in my life. Hopefully I can help by encouraging more of my peers to connect with who they are outside of the game. If ultimately that's what I'm remembered for, I'd be very thankful."
The 30-year-old Thomas wrote that he began auditing a psychology class during the offseason and realized he'd become passionate on the subject.
"Football has serious cognitive risks, but there's still a lot to learn about the extent of them and ways they can be mitigated," he wrote. "The ability to assist with the latest research being done on brain trauma and the pursuit to better understand CTE. To study it not just from the perspective of a psychologist or a clinician, but also as a person who has played for several years at the highest level, is something that excites me. It fills me with a sense of purpose. It's a way to help while growing intellectually that I had been searching for. I know I have so much to learn, but that's what I'm most looking forward to."
A former college basketball player, Thomas was selected in the fourth round of the 2011 NFL draft by the Denver Broncos. After two seasons learning the tight end position, the 6-foot-5 athletic specimen burst out, becoming a go-to red-zone target for Peyton Manning. Thomas scored 24 touchdowns combined in 2013 and 2014, earning Pro Bowl nods each season.
Thomas was traded to Miami in 2017, playing in 14 games and recording three TDs before being placed on injured reserve in December with a foot injury. He was released in March.