any of a group of hormones that primarily influence the growth and development of the male reproductive system.

The predominant and most active androgen is testosterone (q.v.), which is produced by the male testes. The other androgens, which support the functions of testosterone, are produced mainly by the adrenal cortex—the outer substance of the adrenal glands—and only in relatively small quantities. Trace quantities of androgens are found in the female blood plasma. It is believed that the adrenal glands produce most of these small amounts. The ovaries, which normally secrete the female hormones known as estrogens, also produce minute amounts of androgens.

In the male, the interstitial cells of Leydig, located in the connective tissue surrounding the sperm-producing tubules of the testes, are responsible for the production and secretion of androgens. In male animals that breed only seasonally, such as migratory birds and sheep, Leydig cells are prevalent in the testes during the breeding season but diminish considerably in number during the nonbreeding season. The actual secretion of androgens by these cells is controlled by luteinizing hormone (LH) from the pituitary gland.

Androgens are needed for the development of the male reproductive system. Without injections of testosterone, males that have been castrated prior to adolescence and sexual maturity do not develop functioning adult reproductive organs. Androgens given to normal males tend to increase the size of the reproductive organs; castration performed on males that have already reached maturity causes the organs to shrink and to stop functioning. Androgens are also necessary for the formation of sperm cells and for the maintenance of sexual interest and behaviour.

Other effects of androgens upon the male body are diversified. The growth of pubic hair and of facial and chest hair and the regression of scalp hair, or baldness, are influenced by androgens. During adolescence, androgens lengthen and thicken the male vocal cords (vocal cord), causing voice deepening; they also enhance bone growth and increase the number and thickness of muscle fibres in the male body. Other growth patterns that androgens stimulate are kidney weight and size, the increase of protein in bone tissue, the regeneration of red blood cells, the presence of pigments in the skin, and the increased activity of sweat and sebaceous (oil-producing) glands.