Male Sex Hormones

Male sex hormones are also called androgens. The main androgen is testosterone, and an additional androgen is dihydrotestosterone. Testosterone is secreted mainly by the testes, but also a little bit by the adrenal cortex. In some tissues, testosterone is chemically changed into dihydrotestosterone in order to have an effect.

Testosterone is produced in response to tropic hormones. Remember, the hypothalamus releases GnRH, and that stimulates the anterior pituitary to release LH and FSH (the "gonadotropins"). LH gets the testosterone secretion going, and together, LH and FSH promote spermatogenesis.

Testosterone secretion is not continuous throughout the entire life of a male; just after puberty. Testosterone secretion prenatally determines the sex of the fetus. Shortly after birth, testosterone secretion just about stops. Then, during "puberty," testosterone secretion resumes and is maintained for the rest of the life of the male. During the adult male life testosterone secretion is maintained consistently. So, testosterone is only secreted to:

determine the sex of the child (and form the male reproductive organs properly)
enable reproductive function

Based on these two functions, problems in testosterone levels would prevent either the development of appropriate reproductive organs or lack of fertility. The specific effects of testosterone that "enable reproductive function" during puberty and from then on (number 2 above) are the development of the primary and secondary sex characteristics.

Male Primary Sex Characteristic:

Enlargement of the testes (and with it the production of spermatozoa)

Male Secondary Sex Characteristics:

(as found on p. 850 of your book)

increased body hair growth (and sometimes the opposite, baldness, too)

larynx enlargement and thickening of the vocal folds to lower voice pitch

thickening of the skin

increased muscular growth and change in body shape

thickening and strengthening of the bones

additional, less visible traits:

	increased rate of cellular metabolism
	increased production of RBCs (that's why males have more per cubic centimeter)
	and your book says that testosterone also has some effects on the brain, although exactly what they are talking about postnatally, I am not sure of...