Testosterone: What it is and how it affects your health

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. Learn all about the sex hormone here, including its primary benefits.

What is testosterone?

Testosterone is a sex hormone. Hormones are the body's chemical messengers. They travel from one organ or another place in the body, usually through the bloodstream, and affect many different bodily processes.

Testosterone is the major sex hormone in males. It is essential to the development of male growth and masculine characteristics.

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.

What does testosterone do?

Testosterone plays multiple important roles in the body, such as:

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.

Testosterone and women

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 brain for that to occur.

Can you have 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 artificially 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 special type of 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.

Can you 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. Also, as men get older, their livers make more sex hormone binding globulin (SHBG), which binds to testosterone circulating in the bloodstream.

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, problem with function of pituitary gland or adrenal glands may lead to reduced testosterone production. Affected women 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) 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 gland or hypothalamus — tumors, medications (especially anabolic steroids, 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

Testosterone therapy is approved for the treatment of delayed male puberty and abnormally low production of testosterone secondary to malfunction of the testes, pituitary or hypothalamus.

Men may be eligible for testosterone therapy when they have significantly low levels of active (free) testosterone and symptom 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. However, the wisdom and effectiveness of testosterone treatment to improve sexual function or cognitive function among postmenopausal women is unclear.

What are the risks of testosterone therapy?

Some men and women experience immediate side effects of testosterone treatment, such as acne, disturbed breathing while sleeping, breast swelling or tenderness, or swelling in the ankles. Doctors also watch out for high red blood cell counts, which could increase the risk of clotting.

Testosterone therapy does not appear to increase the risk of prostate cancer, but it can stimulate the growth of prostate cancer cells. Because prostate cancer is so common, doctors tend to be leery of prescribing testosterone to men who may be at higher than average risk of having undiagnosed prostate cancer.

For men with low blood testosterone levels and symptoms most likely caused by a low level, the benefits of hormone replacement therapy usually outweigh potential risks. However, for most other men it's a shared decision with your doctor.

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. It's best to measure free testosterone levels in the morning. Even with an abnormally low level that is replicated on a repeat test, the decision to begin testosterone replacement therapy and the proper dose requires a careful conversation with your doctor.

Image: Zerbor/Dreamstime

About the Reviewer

photo of Howard E. LeWine, MD

Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing

Dr. Howard LeWine is a practicing internist at Brigham and Women’s Hospital in Boston, Chief Medical Editor at Harvard Health Publishing, and editor in chief of Harvard Men’s Health Watch. See Full Bio
View all posts by Howard E. LeWine, MD


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