TL:DR - The professional standard of treatment for being transgender is counseling and assistance in transitioning.
Well, assuming you're actually interested in an answer, let's start with some definitions.
A mental illness, or mental disorder, is defined by the DSM-5 (basically the handbook of the mental health field) as:
"A mental disorder is a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning.
Being transgender, does not, in and of itself, constitute a "clinically significant disturbance in an individual's cognition, emotion regulation, or behavior" as you can be a reasonably well adjusted, emotionally stable, well-behaved person, regardless of your gender status (or you can be a total jackass regardless as well, but that's a separate story).
There IS a related condition, known as "gender dysphoria" which the DSM-5 also explains. "For a person to be diagnosed with gender dysphoria, there must be a marked difference between the individual’s expressed/experienced gender and the gender others would assign him or her, and it must continue for at least six months."
The DSM-5 continues - "Persons experiencing gender dysphoria need a diagnostic term that protects their access to care and won’t be used against them in social, occupational, or legal areas. When it comes to access to care, many of the treatment options for this condition include counseling, cross-sex hormones, gender reassignment surgery, and social and legal transition to the desired gender."
Thus, the recommended standard of care for the main "mental disorder" related to being transgender is getting counseling and assistance to complete the transition to the gender the individual feels most comfortable in.
Hope that helps you understand (or, if you were just here to troll, helps you focus your rants toward the American Psychiatric Association in particular, rather than "SJWs" in general).