Testosterone
Adult

Testosterone is necessary for normal sperm development. It activates genes in Sertoli cells, which promote differentiation of spermatogonia. It regulates acute HPA (hypothalamic–pituitary–adrenal axis) response under dominance challenge. Androgens including testosterone enhance muscle growth. Testosterone also regulates the population of thromboxane A receptors on megakaryocytes and platelets and hence platelet aggregation in humans.

Adult testosterone effects are more clearly demonstrable in males than in females, but are likely important to both sexes. Some of these effects may decline as testosterone levels might decrease in the later decades of adult life.

The brain is also affected by this sexual differentiation; the enzyme aromatase converts testosterone into estradiol that is responsible for masculinization of the brain in male mice. In humans, masculinization of the fetal brain appears, by observation of gender preference in patients with congenital disorders of androgen formation or androgen receptor function, to be associated with functional androgen receptors.

There are some differences between a male and female brain that may be due to different testosterone levels, one of them being size: the male human brain is, on average, larger.
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