The World Health Organisation produces a document called the "International Classification of Diseases" (the "ICD").
Version 11 is out and you'll all be delighted to know that they have moved "gender incongruence" (their term for being trans) out of the section for mental illness and into the section for sexual health.
Here is section HA61 on "Gender incongruence of childhood" for example:
icd.who.int/browse11/l-m/en#/http%3a%2f%2fid.who.int%2ficd%2fentity%2f344733949
And note the last line:
"Gender variant behaviour and preferences alone are not a basis for assigning the diagnosis."
That's right - gendered behaviour and gender identity are not the same thing.
Same, of course, for HA60 "Gender incongruence of adolescence or adulthood":
icd.who.int/browse11/l-m/en#/http%3a%2f%2fid.who.int%2ficd%2fentity%2f90875286