Steve Jobs’ letter to the Apple community about his health problems seems to have reassured investors — the stock closed up 4.22% in Monday trading.

But medically, Apple’s AAPL CEO raised more questions than he answered.

His eight paragraph message contains remarkably few health-related facts. They’re all contained in these three graphs:

“As many of you know, I have been losing weight throughout 2008. The reason has been a mystery to me and my doctors. A few weeks ago, I decided that getting to the root cause of this and reversing it needed to become my #1 priority.

Fortunately, after further testing, my doctors think they have found the cause — a hormone imbalance that has been ‘robbing’ me of the proteins my body needs to be healthy. Sophisticated blood tests have confirmed this diagnosis.

The remedy for this nutritional problem is relatively simple and straightforward, and I’ve already begun treatment. But, 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. I will continue as Apple’s CEO during my recovery.” (link)

“Cryptic,” is how Dr. William Sherman, a medical oncologist at the Pancreas Center of New York Presbyterian and Columbia University, described Jobs’ breezy summary of his medical condition. “Delightfully vague,” says Dr. Andrew Ko, a medical oncologist at the University of California-San Francisco. That phrase — “hormone imbalance” — tells us neither what hormones are involved nor why they’re misbehaving.

Moreover, Jobs, who is 53, has left several relevant facts out of this account, starting with the malignant tumor that was removed from his pancreas in 2004 — along with his gallbladder, part of his stomach, the lower half of his bile duct and part of his small intestine. See the Whipple procedure diagram below. The details of this operation were first reported by Peter Elkind in
. (See also Why does Steve Jobs look so thin?)

Several experts, none of whom are involved in his treatment, have speculated that the hormone imbalance Jobs describes may be caused by a recurrence of the original cancer — an islet cell neuroendocrine tumor, one of the few forms of pancreatic cancer that can be successfully treated. If Jobs’ first surgery missed some of those cancer cells, that’s not necessarily a death sentence, says Dr. Ko. With proper treatment — using injectable drugs designed to block key hormone receptors — patients with these cancers can live for years.

Jobs told Apple employees four years ago that his cancer was “cured.” And last year, according to a report in the New York Times, he told several associates — including members of Apple’s board — that he was cancer-free. (See here.) He letter Monday said nothing about cancer, one way or the other.

According to the New York Times, Jobs also told the board that he had a second surgery in 2008 — most likely a surgical “revision” or rearrangement of his internal plumbing to address complications stemming from the original procedure (see here.) The purpose of this second procedure, according to the Times, was to correct ongoing digestive difficulties.

Jobs’ open letter suggests that those digestive difficulties have continue to plague him and offers a new theory — confirmed, he says, by sophisticated blood tests — about what might be going on: an easily treated “hormone imbalance.”

There are several problems with this explanation. Not only is it frustratingly imprecise, but it suggests that Jobs’ hormone issues are something his doctors only just discovered — prompted by his decision a few weeks ago to finally get to the root cause of his weight loss.

Hormones — a broad term for any chemical released by cells that affects cells in other parts of the body — perform a wide variety of functions, from stimulating growth and regulating mood to triggering physical changes like puberty and menopause. They interact in complex ways — through cascading reactions and elaborate feedback loops — and are known to get out of whack now and then, most familiarly in post-menopausal women.

Hormones also regulate digestion and metabolism. In fact, the main function of the pancreas is to produce metabolic hormones (chiefly insulin and glucagon) and digestive enzymes that break down food. Given that Jobs lost a large portion of his pancreas in his 2004 surgery, one would expect his digestive enzymes to be affected — a condition that can be effectively treated with enzyme replacement therapy.  Hormone deficiencies are also common after a Whipple procedure, but they are usually detected and treated early. The mystery, says UCSF’s Dr. Ko, is what — besides a tumor — could cause a hormone imbalance this late in the game.

“If someone is losing weight, you do a workup of his pancreas, you do a workup for diabetes, a workup for hyperthyroidism,” says Columbia’s Dr. Sherman. “Maybe it wasn’t abnormal enough to say so at first, and maybe now it is so a diagnosis can be made.”

What strains credibility — and sounds too good to be the whole story — is that the issue was first raised and the cause discovered only a few weeks ago. If it is something as simple and straightforward as a nutritional problem caused by a hormone imbalance, says Dr. Ko, “I doubt his doctors would have missed it all this time.”

What seems more likely is that Jobs, a man who knows something about controlling the message, is telling us a story as carefully crafted as any Apple product. He has said as little as possible about his medical condition — just enough to calm the waters roiled by his decision to skip this week’s Macworld. And he said it before the markets opened on the eve of Apple’s last Expo — just in time to allow the thousands of reporters, analysts, developers and fans descending on San Francisco to “relax,” as he writes, “and enjoy the show.”

What’s really going on, we still don’t know.

–With reporting by Alyssa Abkowitz.

[Diagram courtesy of the Mayo Clinic]