Steve Jobs, Apple’s iconic chief executive and co-founder, Monday addressed the rumors surrounding his weight loss and decision to forgo delivering the keynote at the Macworld Conference & Expo that begins today.

The issue, he said in a written statement, is a “hormone imbalance” that is sapping him of sufficient protein. The “nutritional problem” is being addressed and Mr. Jobs said he will continue as chief executive during a recovery expected to stretch into the late spring.

Scant information about Mr. Jobs’ health has been disclosed since he was diagnosed with pancreatic cancer five years ago. His gaunt appearance during last fall’s Macworld keynote stirred widespread comment.

In his statement, Mr. Jobs said: “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.” In a separate statement, Aple’s board of directors said that “if there ever comes a day when Steve wants to retire or for other reasons cannot continue to fulfill his duties as Apple's CEO, you will know it.”

In morning trading, shares of Apple climbed $3.67, or 4 percent, to $94.42.

The keynote duties usually handled by Mr. Jobs were handed to Philip Schiller, senior vice president of worldwide product marketing, for this week’s Macworld conference, a venue where Apple traditionally unveils new products. In December, the company announced that Mr. Jobs would not be delivering the keynote and that this would be the last time the company participates in the Macworld conference, which serves as a Woodstock for Apple devotees. Among the most persistent of the flurry of product rumors preceding this conference calls for Apple to roll out an iPhone “nano,” slimmer version of its popular smartphone.

In his statement, Mr. Jobs said blood tests confirmed the hormone imbalance diagnosis. In closing, he said, “So now I've said more than I wanted to say, and all that I am going to say, about this.”