I'm of mixed feelings whether it is a mental illness or a birth defect. It is doubtful we'll ever be able to know unless we determine the environmental or genetic factors that cause it.

Regardless, transsexualism is the only example I can think of where somebody believes they are something they are not, and our society thinks the appropriate thing to do is to surgically and chemically mutilate the patient's body to force the body to conform with their delusion.

If a person of normal height believed they were a dwarf would we shorten their legs by 2 feet to further their illness, or would we try to help them understand they weren't a dwarf?