Thomas, who wrote a retirement letter on The Players' Tribune, said he will pursue a doctorate in psychology.
"I'm ready to admit that I'm okay with giving up the identity of 'Julius the football player,'" Thomas wrote. "I'm O.K. with not being recognized as an NFL athlete -- because I'm more than that. 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 added that he began auditing a psychology class during the offseason and realized he has become passionate on the subject.
After considering signing with another team before training camp, Thomas decided it was time to call it quits.
"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."
The 6-foot-5, 262-pound Thomas played football and basketball at Portland State before being selected in the fourth round (No. 129 overall) of the 2011 NFL Draft by the Denver Broncos.
Thomas played with the Broncos from 2011-14 before signing a free-agent contract with the Jacksonville Jaguars, and then finished his career last season with the Dolphins.
Selected to the Pro Bowl in 2013 and 2014, Thomas caught 226 passes for 2,406 yards and 36 touchdowns in his career.