Shane Carwin announces retirement from MMA

Former UFC interim heavyweight champion Shane Carwin has ended his two-year struggle to return to the cage. Carwin announced his retirement from the sport of mixed martial arts on Tuesday night.

"Officially retired [today]," he wrote on Twitter. "Thank you to my family, friends and fans! #dreambig GOD BLESS!!!"

Once one of MMA's most feared punchers, Carwin improbably shot to UFC stardom on the strength of a violent string of first-round knockouts, culminating in a brutal victory over Frank Mir to win the UFC interim heavyweight strap in less than four minutes at UFC 111.

The former collegiate standout nearly unified the belts four months later, savagely battering heavyweight champion Brock Lesnar for the opening five minutes at UFC 116 before gassing out in the second round and losing via arm-triangle choke. Following neck surgery, Carwin dropped a lopsided unanimous decision to Junior dos Santos in a heavyweight title eliminator at UFC 131. That fight would ultimately be his last.

Carwin spent much of the next two years recovering from back surgery. In mid-2012 the American was selected to coach The Ultimate Fighter 16 opposite Roy Nelson, however a severe knee injury led to the fight's cancellation, as Carwin underwent another round of surgery and rehab.

Carwin retires at the age of 38 with an MMA record of 12-2, having finished all 12 of his victories via knockout (7) or submission (5). From 2008 to 2011, Carwin racked up a 4-2 record inside the UFC, winning his first four but losing his final two.