After 17 years in the game, Chael Sonnen has retired from the sport of mixed martial arts.

The former UFC two-division title contender, who also serves as an analyst for Fox Sports 1 and hosts UFC Tonight, announced his retirement during Wednesday’s edition of UFC Tonight. News of Sonnen’s retirement comes just one day after he was forced to withdraw from a UFC 175 bout with Vitor Belfort after testing positive for anastrozole and clomiphene—two non-steroidal, anti-estrogen medications commonly given to women.

Sonnen began his MMA career in 1997 and started off with a win in his pro debut before turning to developing his amateur wrestling career. After becoming a NCAA Division I All-American and winning awards in the Greco-Roman circuit, Sonnen returned to MMA in March 2002 and never looked back. Sonnen’s first UFC run included fights with Renato “Babalu” Sobral and Trevor Prangley, the latter of whom he defeated by unanimous decision. But fans got their chance to see what Sonnen could really do when he got to the WEC.

After losing to Paulo Filho, Sonnen defeat Bryan Baker before avenging the loss to Filho in a non-title middleweight bout. Sonnen’s UFC return, which came at UFC 95, resulted in a submission loss to Demian Maia, but wins over Dan Miller, Yushin Okami and Nate Marquardt put him in line for a UFC middleweight title challenge against then-champion Anderson Silva. Sonnen would go on to challenge SIlva for four and a half rounds before he was submitted by Silva. Sonnen went on to lose a rematch against Silva and failed in a light heavyweight title bid against Jon Jones as well.

Sonnen coached opposite rival Wanderlei Silva on the recently concluded The Ultimate Fighter: Brazil 3 reality series. The two were to meet after the season concluded, but Wanderlei was first unable to compete due to injury and ultimately bowed out of the bout after failing to appear for a drug test. Belfort stepped in to replace Wanderlei, but Sonnen was then removed from the card after failing his drug test.

Sonnen hangs up his gloves at the age of 37 with an overall record of 28-14-1 and a 6-5 mark in his most recent run with the UFC.