UPDATE 3-Apple's Jobs reassures investors about his health

(Adds analyst, doctor quote, updated share price)

By Sinead Carew

NEW YORK, Jan 5 (Reuters) - Apple Inc (AAPL.O) Chief Executive Steve Jobs sought to soothe investor concerns about his health on Monday, saying his weight loss was caused by a hormone imbalance that is relatively simple to treat.

Shares of Apple rose more than 5 percent in midday trade, as Jobs, a pancreatic cancer survivor, said he would remain CEO during his recovery and that his doctors expect him to regain his weight by late spring.

"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," Jobs said in a statement, breaking his silence about his health for the first time in months.

The statement followed widespread investor concern about the executive's health, after Jobs decided not to give the keynote speech at Apple product showcase Macworld this week. Speculation about his health resurfaced in June 2008, when Jobs appeared markedly thinner at an Apple event.

Jobs is seen as the driver of Apple's successful products, including Macintosh computers, iPod media players and iPhones.

He acknowledged he had been losing weight throughout 2008 and that his doctors determined "a hormone imbalance that has been 'robbing' me of the proteins my body needs to be healthy."

He said the remedy for "this nutritional problem is relatively simple and straightforward" and that he has begun treatment.

Investors who had been unnerved by Jobs's gaunt appearance last year, and the lack of a clear explanation from the company, heaved a sigh of relief.

Pacific Crest Securities analyst Andy Hargreaves said the announcement did allay his and many investors' concerns about Jobs' health, but said he doesn't expect the whispers to stop.

"Is it going to put the fears to rest? No ... just based on experience I would expect there to continue to be rumors on his health deteriorating. But this is pretty definitive," he said.

Apple's board said in a separate statement that Jobs has its full support during his recuperation.

Jobs said the cause of his weight loss had been a mystery to himself and his doctors until he decided a few weeks ago that getting to the root of the problem was his top priority.

Dr. Stuart Weiss, an endocrinologist at New York University Langone Medical Center said the announcement left some questions unanswered.

"With the information he's given, it's very difficult to give a precise diagnosis," he said.