The WHO medical manual says that only crossdreaming crossdressers can become tranwomen, not the ones that do not get aroused by the idea of being a woman. How did sexual arousal become such a sin?
I have spent some time looking at the American DSM manual. The American psychiatric manual say a lot about how some psychiatrist try to draw the line between different types of cross-gender identification.
Under "Mental and behavioural disorders" there is a category called "Disorders of adult personality and behaviour", where you will find both transsexualism and crossdressing/crossdreaming categorized as mental illnesses.