I could be completely misunderstanding you here, but by 'self-identification' are you referring to people who are physically male but identify as female, or the other way around? And then you're asking how that's different from a 'mental health issue' - with the next problem being, how do you define 'mental health issue'? Doesn't everyone have at least some kind of 'mental health issues', or were you thinking of something more specific? They would likely be more inclined to argue that they have a physical issue, though - changing their body to fit their mind is possible to a large extent even if this is controversial to some, the other way around might be more of a challenge.