After weeks of Apple telling journalists that Steve Jobs was pulling out of Macworld for political reasons, it turns out he does have health problems after all.

He said today that he is suffering from a "hormone imbalance", which he says is “robbing me of the proteins my body needs to be healthy".

So what is this mystery illness?

The Apple chief executive's medical history may hold some clues. In October 2003, he was diagnosed with pancreatic cancer, which usually kills sufferers very quickly. But Mr Jobs suffered from neuroendocrine cancer, a rare and, luckily for him, treatable form.

In 2004, he had the "Whipple procedure", which removes the head of the pancreas, where his cancer was, and the duodenum, part of the small intestine.

Martyn Lobley, GP and Times Columnist, said that that anyone who has lived for four years after being treated for pancreatic cancer is both lucky and likely to have been cured of the disease.

But the rare form of pancreatic cancer Mr Jobs suffered from affects the hormone-producing tissue of the organ. A few hormones excreted by the pancreas, if over-produced, could indeed cause problems with digestion. So one suggestion is that this imbalance is a lingering after-effect of his major surgery.

However, Dr Lobley said that the vague and simplistic language used by Mr Jobs to describe his condition does not really reveal much at all. For example, the idea that he is losing "proteins" may mean that he has a bowel condition unconnected to his previous surgery or cancer. Other doctors were similarly at a loss as to what the mystery illness could be.

All this means that speculation and rumours will continue to run rife. Exactly what Mr Jobs says he was trying to head off by making today’s announcement.

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