There has been a lot of discussion about terminology, and words like "intersex", "transsexual", and phrases like "disorders of sexual development". Whether transsexuality is rightly classified as a mental illness or not.
Some of my thoughts, trying to regularise the matter, and fit in with the currently understood definition of "mental illness".
Intersex conditions can reasonably be called disorders, at least, inasmuch as they affect fertility, sexual function, continence etc. They are disorders the way albinism is a disorder. Some might call it a "natural variation", though.
The typically Celtic red hair/fair skin combination, universally recognised as a "Natural variation" could also be called a disorder, as it has real effects on health under some circumstances. It has significant health risks in high-sunshine climates, sort of Albinism-light.
Many Intersexed people who don't see themselves as "disordered" in any way, just a bit unusual, like people with blue eyes, object to the phrase "Disorders of Sexual Development", but they don't have a say in it. The Authorities Have Spoken. As usual.
As above, so below. I've been involved a little on a CAH (Congenital Adrenal Hyperplasia) support forum - CAH is an Intersex condition that masculinises females. And the majority of women that have it insist that their somatic anomaly is no worse than can be gotten from PCOS (Polycystic Ovarian Syndrome), which is not an Intersex condition, and so CAH shouldn't be either. They're not "Intersexed", they have no connection with those "weirdoes and freaks" who have "gender issues". *SIGH*
And some who have other Intersex conditions want nothing to do with those weirdoes and freaks", the Transsexuals, who have "gender issues". *SIGH* again.
My view is that Transsexuality per se is caused by cross-gendered neurology. And I classify it as one of many different Intersex conditions because of that. So it shouldn't be classed as a "mental illness".
On the other hand... the distress caused by severe transsexuality should be so classified, the same way that distress caused by other purely physiological conditions, some congenital, some not, should be. The distress over having the wrong-shaped body for the brain and mind is similar to the distress caused by removal of breasts due to mastectomy, or penectomy due to cancer or accident. And to the extent that that distress can be alleviated, and functionality restored, by surgery and/or hormonal treatment, then it should be. To the extent that it cannot, psycho-therapy may be useful in some cases to help the sufferer (and I choose that word advisedly), cope as best they can with it.
I find it monstrous that children who are arbitrarily surgically modified to have an appearance that conforms to genital norms, whether intersexed or not (and in the past, there have been some dark hints by surgeons that some normal boys were castrated purely because the parents wanted a girl child), and whose mind is not bigendered enough to be content in the new role, are labelled as "mentally ill". It seems the height of inhumanity that they be told late in life that psychotherapy to help them accept what others have tried to force them to be is their only option, that corrective surgery is unavailable to them because.... well, just because.
It is only slightly less monstrous that transsexuals should be treated the same way, especially since their discomfort is so intense that so many have "co-morbidities", ranging from chronic depression, through substance-abuse and other self-destructive behaviour, all the way to suicide.
It is inhuman that various religious groups have either done their best, but been ill-advised by superstitious psychiatrists (as in the Catholic church), or have adopted an anti-Science mentality and so are deaf to medical evidence. They see a physiological problem as a moral one, just as those born with harelip were once thought to be the Spawn of Satan.