Jobs revealed this latest twist in his medical saga Wednesday in a letter to employees posted on Apple's site. He deputized chief operating officer Tim Cook to run the company's day-to-day operations while he took off to "focus on my health" and remove the distraction that his condition has been for Apple.

"During the past week I have learned that my health-related issues are more complex than I originally thought," Jobs wrote, adding that, "As CEO, I plan to remain involved in major strategic decisions while I am out."

In his previous letter, Jobs blamed his weight loss on a hormonal imbalance requiring a "relatively simple" treatment.

Apple declined to comment or answer questions beyond the release of the letter.

The announcement was made after the market closed on a day when Apple's shares fell $2.38 to $85.33, a loss of nearly 3 percent. After-hours trading was halted temporarily, and when it resumed the share price fell 7 percent to the $79 range.

Microsoft co-founder and philanthropist Paul Allen issued a statement wishing Jobs a speedy recovery.

"He has made extraordinary contributions to the personal computer industry, and I really enjoyed working with him in the early days of the Macintosh," Allen said.

Silicon Valley observers differed over the upshot of the leave.

Good timing

Longtime Apple watcher Tim Bajarin said Jobs' leave comes at the best possible time because all of the company's important products have been introduced, minimizing the effect of his absence.

"I think what happened here is that his doctor said you are going to give 100 percent of your attention to getting well," Bajarin said. "In the long run, Apple will be better off if Steve takes six months off and comes back healthy."

Tech analyst Rob Enderle said Apple is being hurt by the series of revelations that continue to spotlight the lack of an heir apparent and the company's dependence on its iconic co-founder.

"Steve has been the guy who brought the product from concept to pitch, and got an audience excited about it," Enderle said. "He is not able to be the face of Apple, and that is a good part of his job."

Jobs' health has been an issue since 2004, when he revealed that he had undergone surgery to treat an islet cell neuroendocrine tumor - a rare form of pancreatic cancer that is curable if diagnosed early. Jobs said he did not have a more common and deadlier form of pancreatic cancer called adenocarcinoma.

Rumors about health

After his cancer treatment, Jobs unveiled the iPhone and continued to lead Apple to new heights. But as he started to appear gaunt in a series of public appearances during the past year, rumors about his health began to circulate. Those concerns grew after Jobs canceled his speech at Macworld this month. Jobs posted a letter on Apple's Web site on Jan. 5 attributing his weight loss to hormonal imbalance that was robbing his body of nutrients.

Now, by his own admission, his health problem is more complex than he assumed. Foster City hormone specialist Dr. Kent Holtorf said that is not surprising given the subtleties of Jobs' apparent ailment and the personality of the man.

Holtorf said Jobs appears to be suffering from a form of hyperthyroidism that causes his metabolism to run faster, making him thinner but also giving the illusion of greater vigor. Holtorf said high-powered individuals and celebrities tend to minimize the harm in the condition.

"They may think, 'I feel better than I did before,' " Holtorf said. "There may be some denial there."

Unknown cause

Dr. Run Yu, medical director of the Carcinoid and Neuroendocrine Tumor Center at Cedars-Sinai Medical Center in Los Angeles, said the key variable is what is causing Jobs' condition. That is a question neither Jobs nor Apple has answered. Yu said Jobs' statements suggest that the hormonal imbalance could be a symptom of a glucagon-secreting tumor in his pancreas. If so, that condition is potentially treatable by surgery or drugs. Yu said these are rare conditions, and the outcome of the illness is difficult to predict. But, he said, "Under optimal treatment, a patient can live 10 years."

Executive coach Marshall Goldsmith, author of "Succession: Are You Ready?" to be released next month, said that in hindsight it would have been wise for Apple's board of directors to have mapped out a clear succession plan and to have gradually lessened its dependence on Jobs. But now the process must take its course, toward his eventual recovery or a gradual disengagement from Apple because of deteriorating health.

"He is a hugely successful, even iconic CEO, and as long as he is there the company benefits," Goldsmith said. "The bad news about that strategy is that as soon as he leaves, you pay for it."

E-mail Tom Abate at tabate@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle