I've already blogged about how gender identity and basic neural architecture appears to be formed in the womb, and how terribly uncomfortable a Male hormonal balance makes someone with a Female brain (and vice-versa).
Now from the European Journal of Endocrinology comes an Academic Paper that might tell us why.
The Abstract of "Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure" :
Objective Sex hormones are not only involved in the formation of reproductive organs, but also induce sexually-dimorphic brain development and organization. Cross-sex hormone administration to transsexuals provides a unique possibility to study the effects of sex steroids on brain morphology in young adulthood.
Methods: Magnetic resonance brain images were made prior to, and during, cross-sex hormone treatment to study the influence of anti-androgen + estrogen treatment on brain morphology in eight young adult male-to-female transsexual human subjects and of androgen treatment in six female-to-male transsexuals.
Results: Compared with controls, anti-androgen + estrogen treatment decreased brain volumes of male-to-female subjects towards female proportions, while androgen treatment in female-to-male subjects increased total brain and hypothalamus volumes towards male proportions.
Conclusions: The findings suggest that, throughout life, gonadal hormones remain essential for maintaining aspects of sex-specific differences in the human brain.
We already know from the autopsy results of Gooren et al that transsexuals' brain structures differ from their cisgendered counterparts, even without hormone treatment (see referenced paper 9) though this is not stated in the article.
It would seem at least plausible that, for example, a male-pattern brain with a female hormonal environment would have a mismatch : the male-pattern structures require a less dense packing of neurons and larger brain volume in order to work properly,and vice-versa for female brains in a male hormonal environment.
But one thing is certain : sex hormone levels change the brain, and long-term hormone replacement therapy with significantly high dosage for 3 months will change the personality. In the quote below, MF means "chromosomal males with female gender" (like me) and FM the reverse.
In MFs, 3 months of estrogen addition and testosterone suppression resulted in a decline in anger and aggression proneness, sexual arousal, sexual desire, and spatial ability (usually males outperform females) and in an increase in verbal fluency (usually females outperform males) (24, 26, 27). In FMs, 3 months of testosterone treatment was associated with an increase in aggression proneness, sexual arousal, and spatial ability performance, whereas it had a deteriorating effect on verbal fluency tasks (25, 28). These behavioral and cognitive findings in transsexuals following cross-sex hormone treatment are in line with the studies reporting influences of endogenous and exogenous sex hormones on behavior and cognition, as well as on cortical brain activation, in non-transsexual adult humans (29, 30). Whether functional brain changes accompany the anatomical brain changes in transsexuals remains to be elucidated using functional brain imaging techniques.One difference in my case - HRT actually gave me a Libido, I'd never had one before. Now this might be due to psychological rather than physiological factors, but my bet is that I now had at least a partly functional female sex drive rather than a wholly broken male one. My condition appears to involve an abnormally feminised hindbrain, where the body map and other instinctive behaviour arises. We think.
I do know - and blogged about - the perceived changes in my thinking during the mad 3 months whgen my hormone levels went crazy (and I nearly did too). It looks like my perceptions matched reality.
So, what does all of this mean from a Therapeutic viewpoint? First, that anyone with "Gender Issues" had better be sure about them before they DIY and order Hormones over the Internet without a prescription. Second, that doses of HRT used for diagnostic purposes may work well - causing Gender Dysphoria in those that don't have it - but should be closely monitored, and be for less than 3 months. We don't know if the neurological changes are reversible, and we may be playing with fire. Third, that those people who have been on HRT for sufficiently long periods of time are already by any reasonable medical definition of their target gender, regardless of their genital configuration. Their Brains have changed.
Legislators please take note.