The major effects of testosterone is that stimulates the growth of certain organs, promotes protein anabolism, that is, the use of protein to build muscle, skin, and bone, and militates against protein catabolism, or breakdown. Testosterone regulates the production of prostaglandin, which seems to keep prostate growth under control.

The effects of testosterone are most pronounced during puberty. It brings on the enlarged larynx, thicker vocal cords, new body hair, increased muscle mass, and increased oil-gland secretion by the skin commonly associated with puberty. After puberty, levels of testosterone drop gradually in men, with profound effects on physical health and well-being and particularly on mood and libido.

Some males suffer when their bodies produce insufficient levels of testosterone, resulting in a condition called hypogonadism. Hypogonadism can be caused by ailments of the testes, such as testicular injury or infection, Klinefelter's syndrome (a chromosomal abnormality), and/or from disorders of the pituitary and hypothalamus