Testosterone is a hormone found in humans, as well as in other animals. The testicles primarily make testosterone in men. Women’s ovaries also make testosterone, though in much smaller amounts.

The production of testosterone starts to increase significantly during puberty, and begins to dip after age 30 or so.

Testosterone is most often associated with sex drive, and plays a vital role in sperm production. It also affects bone and muscle mass, the way men store fat in the body, and even red blood cell production. A man’s testosterone levels can also affect his mood.

Low levels of testosterone, also called low T levels, can produce a variety of symptoms in men, including:

  • decreased sex drive
  • less energy
  • weight gain
  • feelings of depression
  • moodiness
  • low self-esteem
  • less body hair
  • thinner bones

While testosterone production naturally tapers off as a man ages, other factors can cause hormone levels to drop. Injury to the testicles and cancer treatments such as chemotherapy or radiation can negatively affect testosterone production.

Chronic health conditions and stress can also reduce testosterone production. Some of these include:

A simple blood test can determine testosterone levels. There’s a wide range of normal or healthy levels of testosterone circulating in the bloodstream.

The normal range of testosterone for men is between 280 and 1,100 nanograms per deciliter (ng/dL) for adult males, and between 15 and 70 ng/dL for adult females, according to the University of Rochester Medical Center.

Ranges can vary among different labs, so it’s important to speak with your doctor about your results.

If an adult male's testosterone levels are below 300 ng/dL, a doctor may do a workup to determine the cause of low testosterone, according to the American Urological Association.

Low testosterone levels could be a sign of pituitary gland problems. The pituitary gland sends a signaling hormone to the testicles to produce more testosterone.

A low T test result in an adult man could mean the pituitary gland isn’t working properly. But a young teen with low testosterone levels might be experiencing delayed puberty.

Moderately elevated testosterone levels in men tend to produce few noticeable symptoms. Boys with higher levels of testosterone may begin puberty earlier. Women with higher than normal testosterone may develop masculine features.

Abnormally high levels of testosterone could be the result of an adrenal gland disorder, or even cancer of the testes.

High testosterone levels may also occur in less serious conditions. For example, congenital adrenal hyperplasia, which can affect males and females, is a rare but natural cause for elevated testosterone production.

If your testosterone levels are extremely high, your doctor may order other tests to find out the cause.

Reduced testosterone production, a condition known as hypogonadism, doesn’t always require treatment.

You may be a candidate for testosterone replacement therapy if low T is interfering with your health and quality of life. Artificial testosterone can be administered orally, through injections, or with gels or patches on the skin.

Replacement therapy may produce desired results, such as greater muscle mass and a stronger sex drive. But the treatment does carry some side effects. These include:

  • oily skin
  • fluid retention
  • testicles shrinking
  • decrease in sperm production

have found no greater risk of prostate cancer with testosterone replacement therapy, but it continues to be a topic of ongoing research.

One study suggests that there’s a lower risk of aggressive prostate cancers for those on testosterone replacement therapy, but more research is needed.

Research shows little evidence of abnormal or unhealthy psychological changes in men receiving supervised testosterone therapy to treat their low T, according to a 2009 study in the journal .

Testosterone is most commonly associated with sex drive in men. It also affects mental health, bone and muscle mass, fat storage, and red blood cell production.

Abnormally low or high levels can affect a man’s mental and physical health.

Your doctor can check your testosterone levels with a simple blood test. Testosterone therapy is available to treat men with low levels of testosterone.

If you have low T, ask your doctor if this type of therapy might benefit you.