Brandon meets all the above criteria’s for GID as depicted in the movie.
The International Classification of Diseases (ICD-10) list three diagnostic criteria for Gender Identity Disorder and the case of Brandon Teena falls under the category of Trans sexualism:
Trans sexualism has three criteria’s which are “the desire to live and be accepted as a member of the opposite sex, usually accompanied by the wish to make his or her body as congruent as possible with the preferred sex through surgery and hormone treatment, the transsexual identity has been present persistently for at least two years and the disorder is not a symptom of another mental disorder or a chromosomal abnormality” (ICD-10, 1992)
Causes for this disorder can be hormonal influence, biological factors, social factors and social learning theory and sometimes it is unknown but can be treated with family counselling, seeking professional help in understanding, hormonal therapy such as puberty blocker but the end result is usually gender reassignment which is exactly what Brandon was seeking due to his strong desire to be a