Jobs said he will undergo a "relatively simple" treatment and will remain as head of Apple until he recovers.
"A hormone imbalance ... has been 'robbing' me of the proteins my body needs to be healthy," Jobs said in a public letter, adding, "Sophisticated blood tests have confirmed this diagnosis."
Rumors about his health have intensified since Cupertino, Calif.-based Apple said in December that Jobs would not be making his annual keynote address this week in San Francisco at the Macworld conference. Apple said this year's conference will be its last, with Phil Schiller, an Apple marketing executive, giving the company's presentation.
Adding to the speculation, Jobs, who survived pancreatic cancer, has appeared increasingly gaunt at public appearances.
In his statement Monday, Jobs said, "The remedy for this nutritional problem is relatively simple and straightforward, and I've already begun treatment."
He added, "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."
The company's board released a statement separately, 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."