You can literally Google gender dysphoria and look it up yourself. The diagnosis process consists of a behavioral health evaluation. To even qualify to START hormone replacement therapy, you need a letter of referral from a qualified licensed mental health provider. In order to get that kind of referral letter, you need to have an established relationship with a mental health provider and be able to prove you've been seeing them regularly for quite some time.

You ask for specific sources. My sources are the trans friends and classmates I have who have had gender affirming care and who have been through the process of transitioning. My friends have spoken at length about the years of therapy they've needed and the therapy they STILL currently get because with being transgender comes an alarmingly high amount of discrimination, stigmatism, depression, anxiety, self-harm, PTSD. My best friend had to see a therapist once a week for four years and THEN the therapist finally agreed to write him a referral letter to a multi-disciplinary gender clinic for hormone replacement therapy.

The first line of treatment is literally counseling and therapy...you can Google the gender clinic at Boston children's hospital and read more about what they offer if you want your own sources