There's a real psychological disorder that was diagnosed as such until 2017 is called "Body Dysmorphia Disorder". It's disorder in the brain where the patient sees their body differently and strongly fees that certain parts don't belong and want to cover up or amputate. Not just transgenders, but patients often want to cut off their arm or leg or even "feel" disabled and think their body should match. Transgenderism is a sub-type of this disorder, which has symptoms of wanting to cut off genitalia.