Just as estrogen and progesterone are the female sex hormones, testosterone is the male sex hormone (although women have testosterone levels one-tenth to one-twelfth those of men). Testosterone is the main hormone produced in the testicles and secreted by the testes.

The major effects of testosterone are:

• promotes libido, aggressiveness, and sexual desire;
• 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;
• stimulates sperm production;
• nourishes all the tissues of the male urinary and reproductive systems;
• 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 glands.

Dr. Anthony Karpas, director of the Institute for Endocrinology and Reproductive Medicine in Atlanta, believes that the condition is under-diagnosed. He states, "As many as 20 percent of men over age 50 may be hypogonadal."

Some telltale signs of hypogonadism are:

• loss of sex drive/inability to maintain an erection
• fatigue
• irritability
• depressed mood
• aches and pains in the joints
• dry skin
• osteoporosis
• loss of weight
• absence or regression of secondary sexual characteristics, such as muscle development, deep voice, and hair distribution on the chest and face

Testosterone production is affected by a number of external factors, such as illness, medications, psychological state, obesity, exercise, and lifestyle (smoking and excessive alcohol intake). Factors such as reduced activity, nutritional deficiency, diabetes, and growth hormone deficiency can also contribute to lower levels.