The endocrinology of transsexualism: a review and commentary.

Testicular hormones play a decisive role in the sexual differentiation of the genitalia. There is now also an impressive body of knowledge, gathered predominantly from laboratory animals, of the influence of gonadal steroid hormones on the prenatal/perinatal sexual differentiation of the brain. The well-documented mechanisms in animals have been extrapolated, sometimes dogmatically, to the development of sexual orientation and gender identity/role in humans. In principle, it is doubtful that an animal model of human gender identity can be found. Studies in humans have shown that levels of circulating sex steroids and estrogen feedback on luteinizing hormone do not differ between transsexuals and controls. The only reliable sources of information on hormonal influences on human gender identity formation are clinical syndromes in which the hormonal environment of the fetus has been atypical. Follow-up studies of such patients have provided evidence for possible effects of prenatal sex steroids on brain lateralization, sexual orientation and gender role stereotypes. However, a straightforward hormonal effect on gender identity (self-identification as male or female) has been difficult to ascertain up to the present time.