The news sent Apple stock up more than 4 percent on a down day for much of the market. But Jobs did not say whether the problem was related to the cancer, and some analysts said the health watch may not be over.
The CEO's health is an important issue for any company, but especially for Apple, where Jobs has presided over a decade of huge success. His mix of secrecy and high-design principles, seen in the rollouts of new Mac computers, the iPod music player and the iPhone, has become a trademark.
In a public letter, Jobs, 53, said his thinness had been a mystery even to him and his doctors until a few weeks ago, when "sophisticated blood tests" confirmed that he has "a hormone imbalance that has been `robbing' me of the proteins my body needs to be healthy."
"The remedy for this nutritional problem is relatively simple and straightforward, and I've already begun treatment," he wrote. "Just like I didn't lose this much weight and body mass in a week or a month, my doctors expect it will take me until late this spring to regain it."
Jobs, who co-founded Apple with Steve Wozniak in 1976 at the dawn of the personal computer revolution, left in 1985 and returned as CEO in 1997, slashing unprofitable product lines and helping rescue the company from financial ruin.
Jobs announced in 2004 that he had undergone successful surgery to treat a very rare form of pancreatic cancer -- an islet cell neuroendocrine tumor. The cancer is easily cured if diagnosed early. Jobs did not have a deadlier and more common form of pancreatic cancer called adenocarcinoma.
Even so, fears that Apple would lose his leadership percolated in 2008 as Jobs appeared pale, worn and notably thinner in the face. Apple said he was suffering from a common bug, but The New York Times cited anonymous sources who said Jobs had undergone "a surgical procedure" to address the problem that had caused him to lose weight.
Worries about Jobs intensified after Apple said in December that he would not make his annual keynote address Tuesday at the Macworld conference in San Francisco. It was at Macworld in 2007 that Jobs introduced the iPhone.
Apple said Jobs would not take the stage because this year will mark the company's last appearance at the show, which is run by a separate company. Phil Schiller, an Apple marketing executive, will give the company's presentation instead.
But after Apple's official statements didn't quell the discussion of Jobs' health, the CEO said he released his letter Monday "so that we can all relax and enjoy the show tomorrow."
He closed it by saying: "So now I've said more than I wanted to say, and all that I am going to say, about this."
For Apple investors, it seemed that was good enough. Apple stock rose $3.83, 4.2 percent, to close at $94.58.
Still, the announcement is unlikely to end speculation about Jobs, said Brian Marshall, an AmTech Research analyst. He expects Jobs to step down as CEO this year, most likely remaining an adviser to the company. Marshall said he believes Jobs' departure would cut $10 to $15 from Apple's stock price.
An Apple spokesman declined to comment, and Jobs said in his letter that "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."
Stephen M. Davis, a senior fellow at the Millstein Center for Corporate Governance and Performance at Yale University, said the announcement fits Apple's pattern of "releasing information to shareholders in dribs and drabs."
"It's not a technique designed to win loyalty from investors over the long term," he added. "This is a public company in which millions of investors have entrusted their savings and in which millions of customers also have a stake."
Medical experts not involved in Jobs' treatment said there wasn't enough information in his statement to know exactly what was behind his weight loss.
While a hormone imbalance might be unrelated to his cancer, pancreatic dysfunction would be the top suspicion, said Dr. Richard Auchus, an endocrinologist at UT-Southwestern Medical Center in Dallas.
"The pancreas makes digestive enzymes, and if the pancreas doesn't make enough of those, then people can't digest foods and getting nutrients is a problem," Auchus said. The solution could be as simple as taking pills to replace or augment the enzyme, as is commonly done to treat pancreatic insufficiency in children with cystic fibrosis, Auchus said.
Dr. F. Taylor Wootton, a gastroenterologist and spokesman for the American Gastroenterological Association, said another possible explanation for the weight loss is an overactive thyroid gland. Symptoms include weight loss and fatigue. An overactive thyroid is treated with medication, radioactive iodine or surgery.
Apple's board released a separate statement, saying, "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."