Testosterone is the most important sex hormone (androgen) produced in the male body. It is primarily responsible for producing typical adult male attributes.

At puberty, testosterone stimulates the physical changes that characterize the adult male, such as enlargement of the penis and testes, growth of facial and pubic hair, deepening of the voice, an increase in muscle mass and strength, and growth in height.

Throughout adult life, testosterone helps maintain sex drive, the production of sperm cells, male hair patterns, muscle mass, and bone mass.

Testosterone is produced in the testes and the outer layer of the adrenal glands (called the adrenal cortex); in females, small amounts of testosterone are produced by the ovaries.

While it is commonly perceived that testosterone is not a significant factor in prepubescent male development, testosterone is active long before puberty begins. For example, while a fetus is still in the womb, testosterone and a related substance cause the male genitalia to form.

Testosterone Production

The body carefully controls testosterone production. Chemical signals from two glands in the brain—the pituitary and hypothalamus—tell the testes how much testosterone to produce.

The hypothalamus controls hormone production in the pituitary gland using gonadotropin-releasing hormone (GnRH). This hormone tells the pituitary gland to make follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

 LH orders the testes to produce testosterone. If the testes begin producing too much testosterone, the brain sends signals to the pituitary to make less LH. This, in turn, slows testosterone production.

If the testes begin producing too little testosterone, the brain sends signals to the pituitary gland telling it to make more LH, which causes the testes to make more testosterone.

Physical Symptoms Related to Low Testosterone

Signs of low testosterone in men may include decreased sex drive, erectile dysfunction (ED), lowered sperm count, or increased breast size.

Men also may have symptoms similar to those seen during menopause in women – hot flashes, increased irritability, inability to concentrate, and depression.

Some men may have a prolonged and severe decrease in testosterone production. As a result, they may experience loss of body hair and reduced muscle mass, their bones may become more brittle and prone to fracture, and their testes may become smaller and softer.

Low testosterone production in younger men may reduce the development of body and facial hair, muscle mass, and genitals. In addition, their voices may fail to deepen.

Conditions That Could Cause Low Testosterone

Several specific medical conditions can cause low testosterone. Such conditions are often most evident in younger men.

Some of these conditions are associated with the testes, pituitary, and hypothalamus glands. Other men experience low testosterone levels due to various genetic factors.

The effects of aging on testosterone production – Andropause

As men age, their ability to produce testosterone declines. Some men’s production of LH decreases with aging, which lowers testosterone production.

Moreover, a protein called sex hormone binding globulin (SHBG) increases in older men, reducing the amount of free (unbound) testosterone in the blood available to tissues, such as muscles.

Aging also causes changes in the daily cycle of testosterone production. For example, younger men show a peak of testosterone in the morning, but this finding is blunted in older men.

The decrease in testosterone production as men age is sometimes referred to as andropause.

Testes-Based Conditions

Men whose testosterone deficiency is caused by an abnormality in the testes display increased FSH levels, increased LH levels, and impaired sperm production. These conditions include:

  • Trauma – a direct physical injury to the testes may damage the cells that produce testosterone.
  • Orchitis – testicular inflammation can occur after a post-puberty bout with the mumps (there is a higher risk of infertility than low testosterone).
  • Radiation treatment or chemotherapy – these therapies for other diseases may damage the testosterone-producing cells of the testes.
  • Testicular tumors – treatment of testicular tumors may directly affect testosterone production.

Pituitary/Hypothalamus-Based Conditions

Men whose low testosterone levels result from defects in the pituitary or hypothalamus have a low or low-normal FSH level and low or low-normal levels of LH. These conditions include:

  • Pituitary tumors—abnormal tissue growth in the pituitary gland can disrupt the gland’s normal functioning and interfere with hormone production.
  • HIV/AIDS – viruses or other infectious agents may directly or indirectly affect the hypothalamus, pituitary, or testes and can decrease testosterone levels; as many as 50 percent of men infected with the human immunodeficiency virus (HIV) may have low testosterone.

Genetically-Based Conditions

Men may have low testosterone as a result of chromosomal abnormalities or genetically based conditions. These conditions include:

  • Klinefelter’s syndrome – a genetic condition in which an extra X chromosome is present (about one in every 400 men have this); testosterone production is low to low normal; men with this syndrome also may have markedly reduced bone density.
  • Kallmann’s syndrome – usually a recessive genetic disorder associated with the X chromosome, which occurs in about one of every 10,000 men. A deficiency of GnRH impairs the release of LH and FSH, which decreases testosterone production; men with the syndrome lack the sense of smell; testes do not enlarge at puberty.
  • Prader-Willi syndrome – a genetic disorder characterized by decreased muscle tone in infancy that improves with age, underdeveloped genitals (including undescended testes in boys), and low sex hormone levels. An obsession with food and compulsive eating, also linked with this disorder, may begin before the age of six.
  • Myotonic dystrophy – the most common adult form of muscular dystrophy; this genetic condition only occurs in men and is carried on the Y chromosome; because testicular failure usually occurs around the age of 30 to 40, men may have sons at risk for the disease.

Importance of the medical history

Sometimes, physical symptoms can suggest a medical problem. For example, a man who, as he ages, has a progressive decrease in muscle mass, loss of libido, erectile dysfunction (ED), or reduced sperm count may have low testosterone.

Similarly, a teenager who still has the appearance of a young boy – small testes, penis, and prostate; scant pubic and body hair; and a high-pitched voice – shows clear signs of someone with inadequate testicular function.

There are cases, though, that may involve some medical detective work. Therefore, providing the doctor with a detailed medical history is extremely important. Things that should be discussed include:

  • past or present significant illnesses;
  • all prescription and nonprescription drugs currently being taken;
  • family/relationship problems, such as sexual problems, and
  • any major life events or changes that have occurred.

A family history may also help the doctor pinpoint a genetic basis for the problem.

The doctor can use these clues to identify the correct diagnosis.

Physical examination

During the physical examination, the doctor will look at:

  • the amount and distribution of body hair;
  • presence and degree of breast enlargement;
  • size and consistency of the testes;
  • abnormalities in the scrotum and
  • size of the penis.

Measuring hormone levels

Testosterone levels vary from hour to hour, so the time blood is drawn for testing can affect the results.

However, the generally acceptable range of values is 300 to 1,200 nanograms per deciliter (ng/dl). The highest testosterone levels generally occur in the early morning hours, so doctors often measure testosterone levels at this time.

Testosterone circulates in the blood in three forms:

  • about 30 percent of testosterone is bound tightly to a protein called sex hormone binding globulin (SHBG);
  • about 8 percent is weakly bound to another protein called albumin and
  • about two percent circulates freely in the bloodstream.

Determination of low testosterone may require more than one blood test. An average total testosterone reading may not necessarily indicate a man has normal free testosterone levels.

For example, some men with increased levels of SHBG and low blood levels of free testosterone may have normal levels of total testosterone. Therefore, labs often measure the total testosterone levels and its components.

Other tests

  • Because low testosterone levels may affect bone mass, the doctor may want to assess bone loss through bone density testing.
  • Genetic testing can confirm the presence of an inherited condition.
  • If tests cause the doctor to suspect a problem within the pituitary gland, they may want to examine it to see if a tumor is present. Two examination procedures are most common, and neither penetrates the skin. Computed tomography, or CT, is a computer-assisted X-ray process. Magnetic resonance imaging, or MRI, combines radio waves, high-intensity magnetic fields, and computer technology to produce images of the body’s interior.

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