We've had this discussion. Trans is a mental illness. You can call it something different if you don't like the negative connotation, but if your brain thinks you are a different sex than your body is, your brain is not working correctly.
The question then becomes: what do we do about it? We seem to be as a society in a mode of "encourage it, glorify it, and provide medical ways to mutilate your body," which seems like an odd way to address a mental illness.