Gender identity disorder (GID) and gender dysphoria are separate from similar concepts.
Gender dysphoria is the feeling the sex of their bodies "isn't right" - that it feels wrong, that is should be something else, and/or it makes them extremely uncomfortable. Thus gender dysphoria describes a person's relationship with their body. Many transgender people experience gender dysphoria but many do not. Of those who do not, they are comfortable with their body but uncomfortable with the social gender they are grouped into. People with gender dysphoria usually find it subsides if they change their body into alignment with the "sex" of their brain.
GID is an official mental/emotional disorder specified by the American Psychiatric Assocation's (APA) Diagnosis and Statistical Manual of Mental Disorders (DSM). There are four qualifications required for a person to be diagnosed with GID which can only happen under a qualified psychiatrist:
The last requirement is often overlooked. If a person's gender does not cause the negative stress or prevent them from normal functioning, they do not have GID. This point is also contentious because there are many attributes that can cause us distress or impairment but are not mental disorders like race, religion, or any number of unfortunate circumstances like the death of a loved one or losing one's job. If someone experiences racism that keeps them from performing as well as whites, (as many, many Americans do) we do not say (any longer!) they have a "race disorder." Rather we realize some problems which seem to stem from the individual find their true cause in their surroundings. The current medical consensus is that the people with GID are distressed or impaired because of their own gender, not their society. The consensus is also that some transgender people do not have this internal cause. The American Psychological Association says,
A psychological condition is considered a mental disorder only if it causes distress or disability. Many transgender people do not experience their transgender feelings and traits to be distressing or disabling, which implies that being transgender does not constitute a mental disorder per se.
That's an important distinction. For any given trans person, they may or may not have gender identity disorder and may or may not have gender dysphoria. Sometimes the two overlap, sometimes not. Gender dysphoria refers to body identity and GID refers to mental, emotional, and social functioning.
One way of detecting whether someone claiming knowledge about transgenderism is truly knowledgeable is to see whether they know the difference between gender identity disorder and gender dysphoria. People/organizations like Transgender International Fellowship, Danny Blackwell, and Jerry Leach, and betray their ignorance by equating the two. Another way is to see if they equate all transgender people as having GID, rather than recognizing that some transgender people do not, as both the American Psychiatric and Psychological Associations specify.
Trans & Gender 101 >