When you think of testosterone, what comes to mind? Macho men? Aggressive, impatient, type A behavior? Road rage? Violence?

Testosterone's role in bad behavior is largely a myth. What's more, testosterone plays other important roles in health and disease that may surprise you. For example, did you know that testosterone is a key player in prostate cancer? Or, that women need testosterone, too? There's more to testosterone than guys behaving badly.

Testosterone's Role

Testosterone is the major sex hormone in males and plays a number of important roles, such as:

The development of the penis and testes

The deepening of the voice during puberty

The appearance of facial and pubic hair starting at puberty; later in life, it may play a role in balding

Muscle size and strength

Bone growth and strength

Sex drive (libido)

Sperm production

Adolescent boys with too little testosterone may not experience normal masculinization. For example, the genitals may not enlarge, facial and body hair may be scant and the voice may not deepen normally.

Testosterone may also help maintain normal mood. There may be other important functions of this hormone that have not yet been discovered.

Signals sent from the brain to the pituitary gland at the base of the brain control the production of testosterone in men. The pituitary gland then relays signals to the testes to produce testosterone. A "feedback loop" closely regulates the amount of hormone in the blood. When testosterone levels rise too high, the brain sends signals to the pituitary to reduce production.

If you thought testosterone was only important in men, you'd be mistaken. Testosterone is produced in the ovaries and adrenal gland. It's one of several androgens (male sex hormones) in females. These hormones are thought to have important effects on:

Ovarian function

Bone strength

Sexual behavior, including normal libido (although evidence is not conclusive)

The proper balance between testosterone (along with other androgens) and estrogen is important for the ovaries to work normally. While the specifics are uncertain, it's possible that androgens also play an important role in normal brain function (including mood, sex drive and cognitive function).

Did You Know? Testosterone is synthesized in the body from cholesterol. But having high cholesterol doesn't mean your testosterone will be high. Testosterone levels are too carefully controlled by the pituitary gland in the brain for that to occur.

The Perils of Too Much Testosterone

Having too much naturally-occurring testosterone is not a common problem among men. That may surprise you given what people might consider obvious evidence of testosterone excess: road rage, fighting among fathers at Little League games and sexual promiscuity.

Part of this may be due to the difficulty defining "normal" testosterone levels and "normal" behavior. Blood levels of testosterone vary dramatically over time and even during the course of a day. In addition, what may seem like a symptom of testosterone excess (see below) may actually be unrelated to this hormone.

In fact, most of what we know about abnormally high testosterone levels in men comes from athletes who use anabolic steroids, testosterone or related hormones to increase muscle mass and athletic performance.

Problems associated with abnormally high testosterone levels in men include:

Low sperm counts, shrinking of the testicles and impotence (seems odd, doesn't it?)

Heart muscle damage and increased risk of heart attack

Prostate enlargement with difficulty urinating

Liver disease

Acne

Fluid retention with swelling of the legs and feet

Weight gain, perhaps related in part to increased appetite

High blood pressure and cholesterol

Insomnia

Headaches

Increased muscle mass

Increased risk of blood clots

Stunted growth in adolescents

Uncharacteristically aggressive behavior (although not well studied or clearly proven)

Mood swings, euphoria, irritability, impaired judgment, delusions

Among women, perhaps the most common cause of a high testosterone level is polycystic ovary syndrome (PCOS). This disease is common. It affects 6% to 10% of premenopausal women.

The ovaries of women with PCOS contain multiple cysts. Symptoms include irregular periods, reduced fertility, excess or coarse hair on the face, extremities, trunk and pubic area, male-pattern baldness, darkened, thick skin, weight gain, depression and anxiety. One treatment available for many of these problems is spironolactone, a diuretic (water pill) that blocks the action of male sex hormones.

Women with high testosterone levels, due to either disease or drug use, may experience a decrease in breast size and deepening of the voice, in addition to many of the problems men may have.

Too Little Testosterone

In recent years, researchers (and pharmaceutical companies) have focused on the effects of testosterone deficiency, especially among men. In fact, as men age, testosterone levels drop very gradually, about 1% to 2% each year — unlike the relatively rapid drop in estrogen that causes menopause. The testes produces less testosterone, there are fewer signals from the pituitary telling the testes to make testosterone, and a protein (called sex hormone binding globulin (SHBG) increases with age. All of this reduces the active (free) form of testosterone in the body. More than a third of men over age 45 may have reduced levels of testosterone than might be considered normal (though, as mentioned, defining optimal levels of testosterone is tricky and somewhat controversial).

Symptoms of testosterone deficiency in adult men include:

Reduced body and facial hair

Loss of muscle mass

Low libido, impotence, small testicles, reduced sperm count and infertility

Increased breast size

Hot flashes

Irritability, poor concentration and depression

Loss of body hair

Brittle bones and an increased risk of fracture

Some men who have a testosterone deficiency have symptoms or conditions related to their low testosterone that will improve when they take testosterone replacement. For example, a man with osteoporosis and low testosterone can increase bone strength and reduce his fracture risk with testosterone replacement.

As surprising as it may be, women can also be bothered by symptoms of testosterone deficiency. For example, disease in the pituitary gland may lead to reduced testosterone production from the adrenal glands disease. They may experience low libido, reduced bone strength, poor concentration or depression.

Did You Know? There are times when low testosterone is not such a bad thing. The most common example is probably prostate cancer. Testosterone may stimulate the prostate gland and prostate cancer to grow. That's why medications that lower testosterone levels (for example, leuprolide) and castration are common treatments for men with prostate cancer. Men taking testosterone replacement must be carefully monitored for prostate cancer. Although testosterone may make prostate cancer grow, it is not clear that testosterone treatment actually causes cancer.

Diseases and Conditions That Affect Testosterone

Men can experience a drop in testosterone due to conditions or diseases affecting the:

Testes – direct injury, castration, infection, radiation treatment, chemotherapy, tumors

Pituitary and hypothalamus glands – tumors, medications (especially steroids, morphine or related drugs and major tranquilizers, such as haloperidol), HIV/AIDS, certain infections and autoimmune conditions

Genetic diseases, such as Klinefelter syndrome (in which a man has an extra x-chromosome) and hemochromatosis (in which an abnormal gene causes excessive iron to accumulate throughout the body, including the pituitary gland) can also affect testosterone.

Women may have a testosterone deficiency due to diseases of the pituitary, hypothalamus or adrenal glands, in addition to removal of the ovaries. Estrogen therapy increases sex hormone binding globulin and, like aging men, this reduces the amount of free, active testosterone in the body.

Testosterone Therapy

Currently, testosterone therapy is approved primarily for the treatment of delayed male puberty, low production of testosterone (whether due to failure of the testes, pituitary or hypothalamus function) and certain inoperable female breast cancers.

However, it is quite possible that testosterone treatment can improve symptoms in men with significantly low levels of active (free) testosterone, such as:

Generalized weakness

Low energy

Disabling frailty

Depression

Problems with sexual function

Problems with cognition.

However, many men with normal testosterone levels have similar symptoms so a direct connection between testosterone levels and symptoms is not always clear. As a result, there is some controversy about which men should be treated with supplemental testosterone.

Testosterone therapy may make sense for women who have low testosterone levels and symptoms that might be due to testosterone deficiency. (It's not clear if low levels without symptoms are meaningful; treatment risks may outweigh benefits.) However, the wisdom and effectiveness of testosterone treatment to improve sexual function or cognitive function among postmenopausal women is unclear.

People with normal testosterone levels are sometimes treated with testosterone at the recommendation of their doctors or they obtain the medication on their own. Some have recommended it as a "remedy" for aging. For example, a study from Harvard Medical School in 2003 found that even among men who started out with normal testosterone results noted loss of fat, increased muscle mass, better mood, and less anxiety when receiving testosterone therapy. Similar observations have been noted among women. However, the risks and side effects of taking testosterone when the body is already making enough still discourages widespread use.

The Bottom Line

Testosterone is so much more than its reputation would suggest. Men and women need the proper amount of testosterone to develop and function normally. However, the optimal amount of testosterone is far from clear.

Checking testosterone levels is as easy as having a blood test. The difficult part is interpreting the result. Levels vary over the course of the day. A single low level may be meaningless in the absence of symptoms, especially if it was normal at another time. We need more research to know when to measure testosterone, how best to respond to the results and when it's worthwhile to accept the risks of treatment.

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