Thornton said in a statement released by his foundation Friday. He thanked his former coaches and said he hopes to remain connected to football during retirement.
"My love and passion for the game will never waver, but I am confident that this is the right decision for me at this time," Thornton said in the statement. "Growing up, playing in the NFL seemed farfetched. However, the Lord had greater plans for me than I ever imagined."
Thornton had 809 tackles, five sacks, five interceptions, 11 forced fumbles and one fumble recovery in 121 games. He missed the 2010 season recovering from hip surgery starting on the physically unable to perform list before being placed on injured reserve.
He also thanked the Titans for having faith in his ability beyond just his on-field production, especially in 2010.
The Goldsboro, N.C., native was a walk-on at North Carolina before starting 11 games as a senior. He was selected in the fourth round of the 2002 draft by Indianapolis, where he spent the first four seasons of his career before being signed by the Titans as an unrestricted free agent.