Kenny Florian, the only fighter in UFC history to have competed in four weight classes, announced his retirement from fighting on Thursday night following the TUF Live Finale weigh-ins in Las Vegas.

The 36-year-old has pondered the possibility for months, and ultimately cited a recurring back injury for his decision, saying he refused to compete while his body was compromised.

"It's been very hard," he said of the decision. "You're identity becomes what you do, so it's been tough."

Florian began his career in 2003, and was discovered by the UFC the next year when UFC president Dana White went to scout Drew Fickett for what was soon to be the first season of The Ultimate Fighter.

Despite the fact that Florian lost in a split decision, White was impressed by his toughness and offered him a spot on the show. Florian fought above his weight division to compete as a middleweight and advanced to the finals, where he lost to Diego Sanchez.

His UFC career comprised 17 fights over seven years, and he posted a 12-5 record in that time. All five of those defeats came to eventual champions (Sean Sherk, BJ Penn, Jose Aldo) or No. 1 contenders (Gray Maynard, Sanchez).

The only knock on Florian in his career was his inability to win the big one, as he lost in three opportunities to capture a title. His last fight came against featherweight champ Jose Aldo last October, with Aldo taking a decision with a sweep of 49-46 scores.

His greatest moment likely came in a No. 1 contender fight with Joe Stevenson back in November 2008, a bout which saw Florian blow him out with a first-round submission in just 4:03. He also holds wins over respected veterans like Clay Guida, Takanori Gomi, Joe Lauzon and Roger Huerta. He finishes his career with a 14-6 overall record in 20 pro fights, and a sterling reputation as a true professional.

Florian said he would continue on with his burgeoning commentary career, coaching and plans to open a gym in Los Angeles, where he recently moved.