The APA explained this in explicit terms when it stopped using the term “gender identity disorder” in favor of “gender dysphoria”: “Part of removing stigma is about choosing the right words. Replacing ‘disorder’ with ‘dysphoria’ in the diagnostic label is not only more appropriate and consistent with familiar clinical sexology terminology, it also removes the connotation that the patient is ‘disordered.’”
Gender dysphoria — a state of emotional distress caused by how someone’s body or the gender they were assigned at birth conflicts with their gender identity — is a widely recognized medical condition. If untreated, it can lead to severe mental health issues, including debilitating depression, anxiety, and suicidal ideation.
But the AMA, APA, and other medical experts agree that letting someone transition, which can entail medical treatments like hormone therapy and gender-affirming surgeries, without social stigma is the main treatment for gender dysphoria. In this way, being trans isn’t the medical condition; living as trans is in fact the treatment to the medical condition.
And not all trans people deal with severe dysphoria. It’s barely or not present for some trans people, while it’s mentally excruciating for others.
These facts show that psychological distress and disability aren’t inherent to being trans, so being trans doesn’t meet the definition of a mental disorder (a psychological state that causes significant distress and disability).