Gender identity disorder
Gender identity disorder is a medical diagnosis for a mental disorder according to the DSM IV, which indicates a "strong and persistent cross-gender identification."  In previous versions of the DSM, this had been called "Gender Dysphoria", but now that term is used specifically to refer to a specific psychological criterion of GID which requires that "there must also be evidence of persistent discomfort about one's assigned sex or a sense of inappropriateness in the gender role of that sex." 
In the ICD, this mental disorder remains known as the more common term "transsexualism", which as its first diagnostic criterion requires the individual to have a "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." As with gender dysphoria, homosexuality alone does not clinically satisfy this criterion, but is strongly affirming when in conjunction with other desires to be the opposite sex.
A set of International Standards of Care  guide most physicians, and therapists around the world in a widely accepted medical process that begins with a form of Reparative Therapy, which is intended to dissuade patients from the permanent and irreversible physical alterations that could seriously damage their mental health. Patients are expected, and strongly encouraged to attempt to resolve this disorder without physical intervention;[Citation Needed] however when such therapy has been found to not make a positive impact on the patient's mental health, then the standards indicate physical interventions, which are focused and intended to improve the mental health of the patient, or ease their assimilation into the new role in order to alleviate stress from societal, and cultural pressures against transsexualism.
Motives and models of GID and transsexualism
How this condition relates to homosexuality is hotly debated. At the time of Kinsey's evaluation of human sexual behavior, transsexualism was modeled as being so homosexual that one felt a need to interact with heterosexual members of their own sex.[Citation Needed]
However, the Kinsey model fails to account for a significant amount of transsexuals, who are attracted to people of the opposite gender as their destination gender. As a result, transgender activists claim that sexual orientation and gender identity are independent of one another. 
Critics of this model, such as J. Michael Bailey claim that such a model fails to properly account for evidence that transsexuals can be divided into two separate and distinct categories. One group of males are so naturally effeminate and develop identically to homosexuals in youth that it is considered better for them to be a women, than to continue to struggle and/or fail as men. These naturally effeminate transsexuals contrast with all other transsexuals, who are described as having a form of non-harmful paraphilia, called autogynephilia, whereby their erotic desires revolve around considering themselves as women. This paraphilia then progresses to consume their erotic desires. As the erotic gratification naturally diminishes, it is supplemented by deeper and deeper fantasies and acts, which culminate in the compulsion to actually become the woman that they have imagined themselves to be. Unlike transsexuals who are naturally effeminate these transsexuals with a paraphilia are empirically shown to share a common and frequent rejection from family, friends and society that are outside of the transgender community, as they continue to remain notably masculine and behave differently from natural females, regardless of how much surgical intervention, or behaviorial training they seek.