San Jose, Calif. » In an unprecedented letter to Apple investors and consumers Monday, Chief Executive Steve Jobs revealed that a hormone imbalance is "robbing" his body of proteins, causing him to drastically lose weight and triggering deep concern about his health in recent months.

Jobs said he has begun treatment and will remain head of the Cupertino, Calif., company. The news buoyed Apple's stock, which closed Monday at $94.58, up more than 4 percent.

For months, Jobs and his company fended off questions about his gaunt appearance, saying issues about his health are private. Jobs battled pancreatic cancer four years ago and reportedly told the company's board last year that he was cancer-free. Unlike perhaps any other American chief executive, Jobs is considered central to his company's innovative success.

When Apple announced last month that Jobs would not give the keynote address Tuesday at Macworld in San Francisco, which will be the last Macworld the company will participate in, rumors about the co-founder's ill health spread across the Internet, sending Apple's stock down. Apple Senior Vice President Phil Schiller will take Jobs' place on the big stage.

Jobs, who is loathe to reveal information about his personal life, acknowledged this in Monday's letter.

"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," he wrote. "I've decided to share something very personal with the Apple community so that we can all relax and enjoy the show tomorrow."

Jobs said the treatment, which he did not describe but called "relatively simple and straightforward," will enable him to regain his weight by spring. The condition, Jobs said, was a "mystery to me and my doctors." The diagnosis of a hormone imbalance was made after he underwent "sophisticated blood tests."

Jobs said the treatment will not interfere with his duties as chief executive.

"I have given more than my all to Apple for the past 11 years now," said Jobs, who returned to the company in 1997. "I 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. I hope the Apple community will support me in my recovery and know that I will always put what is best for Apple first."

The pressure for Apple and Jobs to address the issue of his health kept building and needed to be addressed, said Tim Bajarin, president of Creative Strategies.

"There has been pressure from Wall Street, the media, etc. for Apple to clarify Steve's health," he said. "The rumors were flying everywhere and the rumors were wrong. They needed to nip them in the bud."