"Nobody" does? I think a lot of people call themselves "transgender," but count me as one vote for calling myself that at least.

Frankly, as someone who considers her gender identity to have shifted over time... I'm always afraid that if someone does invent a medical "test" for being trans, I'd fail it. Seriously. I just had an MRI recently and was afraid they'd say I had a male brain! (Turns out what they were looking for wasn't that specific, or else nobody said.) I've also seen the brain studies go back and forth so often - and I used to work as a medical reference librarian, so I'm also qualified to say that some of those studies weren't very good - that I'd be scared to death to pin proof of my or anyone's transness on them. I transitioned and am 1000% happier. That's all the proof *I* need, but imagine if the tests had said I don't qualify as truly trans* and forced me to continue living as a man?
Anyway, yeah, personally I prefer "transgender" to an acronym that a) most people won't recognize and b) validates a line of medical inquiry that terrifies me. Sorry.