I'm inclined to follow your lead on that, having no medical background myself.
I was under the impression the medical community was moving away from the "mental illness" model for trans people...is that incorrect?
I get the sense that the migration of thought from the medical community away from "mental illness" has more to do with social activism than anything else. I mean you have people who call themselves "scientists" saying that there is no biological sex, that is that sexual identity varies independently from biology. Yet something like 99.7% of the population regardless of culture and location identifies with the "equipment" they have.
In those rarer cases, it's been traditionally viewed as a mental disorder where the person feels uncomfortable with the sex organs they have, along with a strong belief that they would or do feel more comfortable with having the sex organs of the opposite sex. So society has gotten some things wrong about transgendered people. They've been ridiculed and forced into the closet, when it's something that they can't really help. These people shouldn't be ridiculed any more than a disabled person should be ridiculed. But we also shouldn't treat it as if it's up to their whims that they arbitrarily choose their sex.