NFL tight end Julius Thomas is hanging up his cleats for a doctorate degree.

Thomas is retiring from professional football to pursue a Ph.D. in psychology, the 30-year-old announced in the Players' Tribune on Friday.

Thomas said he will focus his doctorate on "investigating the effects of contact sports on brain trauma and neurobehavioral performance." He will also participate in research to "identify early warning signs of brain disease."

He said he felt particularly called to the study of CTE: "I knew I wanted to help and knew that this would be a great way to help football players, to be that person who could help them understand what we all are at risk of. I felt inside that this was stuff I needed to understand."

Thomas wrote that he began to "take stock" of where he was in his life and had "work to do internally" during the course of his football career.

Julius Thomas felt drawn to studying CTE because of his football career. Chris Humphreys/USA TODAY Sports

The two-time Pro Bowler made the decision "to follow my heart" and pursue football after playing Division I basketball at Portland State and was eventually drafted in the fourth round by the Denver Broncos in 2011. He had his breakout season in 2013, when he scored 12 touchdowns on 788 receiving yards and was selected to his first of two Pro Bowls. But Thomas wrote that he eventually had to "learn to accept that my identity was not my profession," making the decision to study psychology.

"While making the decision to no longer play the game is difficult, I'm also incredibly excited about what's next: studying therapy and becoming well trained in it so that I can help people heal from their emotional and mental pain," he wrote.

After his success in Denver, Thomas joined the Jacksonville Jaguars in 2015 on a five-year, $46 million contract. He was traded to the Miami Dolphins two years into the contract after recording just 736 yards and nine touchdowns in that span. The Dolphins released him in March.

He finishes his NFL career with 2,406 receiving yards and 36 touchdowns.