SAN FRANCISCO — Apple shares climbed yesterday after CEO Steve Jobs said he has a treatable hormone imbalance that will not prevent him from running the company.

The news, at least for one day, quelled investor anxiety about his health and the future of the company he co-founded. In a public letter, Jobs, 53, said a weight loss that left him looking gaunt had been diagnosed with "sophisticated blood tests." The hormone imbalance "has been 'robbing' me of the proteins my body needs," he wrote. Jobs said he will undergo a "relatively simple" treatment and decided to release the information after his decision not to deliver the Macworld Expo keynote address today "set off another flurry of rumors" about his health. Jobs offered few details. It was not clear whether the hormone problem resulted from his treatment for pancreatic cancer several years ago.

"It is hard to know without specific medical information," says Dr. Umesh Masharani, a clinical professor of medicine at the University of California, San Francisco. "It could be recurrence of a tumor, or the consequence of surgery, in which he lost pancreatic fluids. It could be a number of things."

It is also unclear if Jobs gave a more detailed update about his health to Apple's board.

In a statement, the seven-member board said, "Apple is very lucky to have Steve as its leader and CEO, and he deserves our complete and unwavering support during his recuperation." Apple had no further comment.

Investors were thankful for any clarity on Jobs' health. Apple's shares rose 4 percent, to $94.58, in trading yesterday.

"The rise indicates Apple eliminated one little bit of uncertainty," says Mike McGuire, a digital-media analyst at market researcher Gartner. "His health issues are no longer a rumor. The news came directly from him."

But some analysts, such as Broadpoint AmTech's Brian Marshall, remain troubled by Jobs' health and its long-term impact on Apple. Marshall wonders who would succeed Jobs if it became necessary.