On the eve of Macworld, traditionally a spotlight on Apple products, Steve Jobs went public this morning with the reason for his low profile of late—a hormone imbalance that he says is being treated while he remains CEO of Apple (NSDQ: AAPL). The news emerges after months of speculation about the prostate cancer survivor’s health, talk that ramped up last month after word that Jobs would not give his traditional Macworld keynote and has been hampering Apple’s stock. In a letter released as a press release as the market opened Monday morning, Jobs said it was that speculation that prompted him to make this disclosure.

The public acknowledgment that Jobs is struggling with a new health problem is Apple’s effort to release the pressure building up from the mix of rumors that initially kicked into gear with his gaunt appearance earlier this year and the Tuesday start of a Jobs-less Macworld. It’s an announcement that could have been made last month—and, given the way that Jobs and Apple have traded on his reputation to pitch the company, it should have been. Yes, he has the right to privacy if he and the board believe the issue is not material to his ability to function as CEO, but the speculation was a serious distraction that ran the risk of overwhelming the messages the company wants to get across. So far, the stock is up by about 3 percent in early trading. More after the jump.

Jobs wrote: “Unfortunately, my decision to have Phil deliver the Macworld keynote set off another flurry of rumors about my health, with some even publishing stories of me on my deathbed. I’ve decided to share something very personal with the Apple community so that we can all relax and enjoy the show tomorrow. As many of you know, I have been losing weight throughout 2008. The reason has been a mystery to me and my doctors. A few weeks ago, I decided that getting to the root cause of this and reversing it needed to become my #1 priority. Fortunately, after further testing, my doctors think they have found the cause—a hormone imbalance that has been ‘robbing’ me of the proteins my body needs to be healthy. Sophisticated blood tests have confirmed this diagnosis.”

His recovery could take until late spring but Jobs plans to remain as CEO, saying in the letter that he “will be the first one to step up and tell our Board of Directors if I can no longer continue to fulfill my duties as Apple’s CEO” and then tried to put the lid on the discussion: “So now I’ve said more than I wanted to say, and all that I am going to say, about this.”

In a statement released at the same time, the Apple board of directors said: “As we have said before, if there ever comes a day when Steve wants to retire or for other reasons cannot continue to fulfill his duties as Apple’s CEO, you will know it. Apple is very lucky to have Steve as its leader and CEO, and he deserves our complete and unwavering support during his recuperation. He most certainly has that from Apple and its Board.”

Photo Credit: Danny Novo