Apple co-founder and chief executive Steve Jobs posted an open letter to customers on his company's Web site today in which he says that he's being treated for an unspecified "hormone imbalance" that has caused severe weight loss and kept him out of the public eye. Jobs's letter is short on details, making it difficult to figure out the actual disorder that he's battling, says Robert Lustig, professor of clinical pediatrics at the University of California, San Francisco. "There are three medical threads that run through the [letter], but unfortunately they don't form a strong cable," he says.

In the letter, addressed to the "Apple Community," Jobs, who is not scheduled to address the MacWorld conference in San Francisco this week as he has done regularly since returning to the company in 1997, says that "a hormone imbalance has been robbing me of the proteins my body needs to be healthy." Jobs, who was reportedly successfully treated for early-stage pancreatic cancer in 2004, notes that the "remedy for this nutritional problem is relatively simple and straightforward," but does not spell out what it is.

Lustig says it is unclear what type of illness might cause this protein deficiency, because it is unknown whether Jobs cannot properly absorb and process proteins or whether his body has stopped making them. In addition, he says, a hormonal imbalance would suggest that there's a problem with his endocrine system (ductless hormone-producing glands—including the pancreas, thyroid and pituitary—that control reproductive capabilities and metabolism as well as bodily growth and development), although endocrine disorders are not generally considered to be "nutritional problems".

Jobs's reference to a "hormone imbalance" in the letter alone is too vague and general to diagnose, agrees Bruce White, a cell biology professor at the University of Connecticut Health Center in Farmington. But he says that if Jobs has a hormone imbalance that is affecting the amount of proteins in his body, it points to a possible problem with production of the hormone insulin (by the pancreas) and the possible onset of type 1 diabetes (which is rare but not unheard of in adults).

"The main regulator of protein synthesis is insulin," White says. "If he's not producing sufficient insulin, [his body] would have trouble transporting amino acids [the building blocks of protein] into his cells."

Jobs's battle with pancreatic cancer (he had an islet cell neuroendocrine tumor removed in 2004 and has since been in remission) adds another piece to the puzzle. If the tumor that Jobs had removed in 2004 had begun to break down prior to the surgery, White says, the tumor's dead cells could have released protease and lipase enzymes that may have damaged beta cells in the pancreas, which produce insulin. But he stresses that it is impossible to accurately diagnose Jobs's condition without more details.

Steve Dowling, an Apple spokesperson, said that the letter is all that the company plans to release on the Apple chief's condition. Speculation has swirled about Jobs's health since he announced last month that he would not appear MacWorld, which is running through January 9. The official reason given for Jobs's absence was that the company is preparing "to wind down its participation in MacWorld," according to the Washington Post.