Hunter met Byrne in life, and had designs to acquire the giant's skeleton after his death, for his museum. However, Byrne tried to thwart Hunter's plans, and after he died at the age of 22, he had made his friends promise to bury him at sea. Hunter's hired hands, unfortunately for Byrne, managed to bribe or intoxicate the funeral party, or so the story goes, and stole the body away to Hunter's back door in the upmarket Leicester Square neighborhood, where in his basement the body was boiled down to a skeleton.
According to the Guardian, an endocrinologist named Márta Korbonits at London's Barts hospital has found a possible genetic cause for Byrne's gigantism. His growth spurt (and subsequent early death) was due to an excess of growth hormone produced by the pituitary gland. Pituitary tumors are the most common cause of gigantism, and today is treated by surgical removal of the tumor.
She studies a genetic condition called familial isolated pituitary adenoma, which is caused by a mutation in a gene called AIP (Aryl Hydrocarbon Receptor Interacting Protein). According to her work, Byrne's mutation (discovered by sequencing DNA from his teeth) came from a common ancestor as a population of people bearing the AIP mutation in Ireland today. As the article says:
Both Holland and McCloskey [a FIPA survivor and film-maker] came from Tyrone and were fascinated by the number of actual giants in the area, and by the way they figured in Irish folklore not as freaks, but as kings, seers and poets.
John Hunter himself was also a colorful character, living in a colorful age. He might be called the prototype of the 'brash surgeon', more interested in showing off their surgical chops (and in Hunter's case - acquiring specimens for his collection) than the comfort of their terrified patients.
A good recent biography of him is Wendy Moore's The Knife Man. In his time (and indeed until the modern age), there was no clear line between medicine and natural history, and many of his famous experiments and observation concern the latter subject.
He was one of the first to experiment with surgical grafting. Among the grafts he performed was that of a freshly-pulled human incisor onto the comb of a rooster. We are told that this required repeated trials–I wonder who donated all those teeth! Eventually, the graft did 'take', and he sacrificed the animal, in order to bisect its comb to see how it connected to the tooth. He found that blood vessels from the rooster had indeed penetrated the tooth pulp: "The uniting of parts of different animals when brought into contact he attributed to the production of adhesive instead of suppurative inflammation, owing to their possession of 'the simple living principle.' " (Encyclopedia Britannica).
After his death, his anatomical collection found its way to the Company of Surgeons. It suffered damage during World War II, but is still on display today in London.