I'm pretty sure that Cassidy suffered from Gigantism rather than Acromegaly. His growth at a young age (6'1" at age 11) before epiphyseal closure is more in line with gigantism. Acromegaly occurs after epiphyseal closure. Both are caused by an excess of Growth Hormone, but the differential diagnosis between the two is the age of onset. Richard Kiel's bio says Acromegaly, but I'd question that. Acromegaly has the large hands and feet, facial structural enlarement changes (forehead, jaw and nose), thickening of the skin, deepening of the voice, but they don't reach excessive heights, because the epiphysis are closed. The onset is typically seen in middle age. The majority of those effected are due to a pituitary adenoma (95% of the cases), the other 5% occurs from a tumor elsewhere in the body that's producing GH. It is completely possible that Cassidy and Kiel continued to have excessive GH after they reached maturity and their height stopped, but endocrinologists would still most likely classify their condition as Gigantism or Pædiatric Gigantism with Adult onset Acromegaly. My great uncle (by marriage) suffered from Acromegaly. Pictures of him in his 20's to early 30's looked normal, but from about 38 to 56, you could see the facial changes and his hands and feet getting larger. He died of heart problems in his early 60's, when I was 12 or 13.
Additionally, Gigantism is thought to be a genetic trait, but 50% of the cases studied didn't share the same gene dupication (thefe'sseveral genes that are culprits). So there's still a lot of conjecture and research going on. Acromegaly, has no genetic component at all, that they've been able to identify. At some point in the years after puberty the adenoma starts up and the growth hormone level goes way up. Cardiac problems, Type 2 Diabetes, severe Sleep Apnia and Hypertension are complications that can occur as a result of suffering from Acromegaly (also severe joint pain, headaches and vision problems). With treatment (surgery, radiation therapy and/or medication, somastatin analogue, GH receptor antagonist), Acromegaly patients can live to a normal lifespan. Untreated, they will die prematurely by 10 or more years.