SAN JOSE, Calif. -- In an unprecedented letter to Apple investors and consumers yesterday, Chief Executive Officer 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.

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

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

When Apple announced last month that Mr. Jobs would not give the keynote address today 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 speak.

Mr. Jobs said the treatment, which he called "relatively simple and straightforward," would enable him to regain his weight by spring. The diagnosis of a hormone imbalance was made after he underwent "sophisticated blood tests."

"So now I've said more than I wanted to say, and all that I am going to say, about this," Mr. Jobs said.