From the NHS -
"Gender dysphoria is a recognized medical condition, for which treatment is sometimes appropriate. It's not a mental illness. Some people with gender dysphoria have a strong and persistent desire to live according to their gender identity, rather than their biological sex."
It's not a mental illness. People need to realize that it's a medical condition.
The BMJ, a leading medical journal, puts it this way.
"Illness is defined as the ill health the person identifies themselves with, often based on self reported mental or physical symptoms. ... Disease, on the other hand, is defined as a condition that is diagnosed by a physician or other medical expert."