After the former world road champion was able to complete fewer than three dozen days of racing for BMC Racing in 2012, he underwent four days of medical tests at the Salt Lake City Health and Fitness Institute under the supervision of the team's chief medical officer Dr. Max Testa.
"The purpose of the tests was to explain his reduced exercise tolerance that has been limiting his performance throughout the season," Testa said.
"At the end of the four days, we determined Thor suffered a post-viral syndrome with secondary myositis – or muscles inflammation. His medical condition is currently improving, and a full recovery is expected for next season."
Norwegian Hushovd expressed his relief at the diagnosis.
"It's a really big relief that I've been fighting all year with these bad sensations on the bike," Hushovd said.
"After a while, you lose some confidence. But now that I've finally found out why I had this weakness, I'm relieved.
"Now I know that I can come back strong for next year."
Hushovd, whose best result was fifth on stage 7 of Paris-Nice in March, did not compete in the Tour de France for the first time since 2000 and withdrew from the Giro d'Italia after five stages.
He also had to miss the London Olympics.
He is to begin training again on October 1 in preparation for 2013.
"Right now, I'm just keeping the muscles active," he said. "Yesterday I rode my bike and did some core training.
"So now I'll just be trying to stay active until the first of October so I don't lose too much fitness."