Gigantism is a growth disorder caused by an overproduction of pituitary hormones, generally human growth hormone, during adolescence. If the disorder continues into adulthood, it becomes acromegaly, a far more serious disorder. The overproduction of hormones can be the result of several causes, such as a genetic mutation or a hormone producing tumor.
For an oversized person to be classified with gigantism, they must do more than merely be in the upper 1% of individuals of the same age, sex, and ethnic background. The diagnostic criteria is that the patient must be several standard deviations taller and heavier than others from a similar background. A standard deviation is the measurement which covers the difference between the average of a group and contains almost all of the individuals in the group. For North American adult males, the standard deviation comes in at about 6 inches (with the average being 5'8" and the vast majority being between 5'2" and 6'2". As such, someone 18 inches over the average (over 7'2") could be a candidate.
People who have gigantism have several health problems. They often have oversized organs, including the heart, which are subject to have internal defects. They also tend to have problems with the bones of the leg and feet which can lead to infection.