Source: The Hormone Foundation - The Public
Education Affiliate of the Endocrine Society
Testosterone and Its Function
Testosterone is the most important sex hormone (otherwise known as androgens) produced in the male body. It is the hormone that is primarily responsible for producing the 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 in 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 major 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.
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The body carefully controls the production of testosterone. 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 by means of 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 the production of testosterone. 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.
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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 be more brittle and prone to fracture, and their testes may become smaller and softer. In younger men, low testosterone production may reduce the development of body and facial hair, muscle mass and genitals. In addition, their voices also may fail to deepen.
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Conditions That Could Cause Low Testosterone
There are a number of specific medical conditions that can cause low testosterone. Often, such conditions are most evident in younger men. Some of these conditions are associated with the testes, pituitary gland and hypothalamus gland. Other men experience low testosterone levels as a result of 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, which reduces the amount of free (unbound) testosterone in the blood that is 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.
Men whose testosterone deficiency is caused by an abnormality in the testes often 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 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.
Men whose low testosterone levels result from defects in the pituitary or hypothalamus generally have a low or low-normal FSH level and low or low-normal levels of LH. These conditions include:
- Pituitary tumors - the growth of abnormal tissue in the pituitary 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 hypothalmus, 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.
Men may have low testosterone as a result of chromosomal abnormalities or genetically-based conditions. These conditions include:
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- 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.
Diagnosing Low Testosterone
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, it is extremely important to provide the doctor with a detailed medical history. Things that should be discussed include:
- past or present major 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 also may help the doctor to pinpoint a genetic basis for the problem.
The doctor can use these clues to identify the correct diagnosis.
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 at which 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). Generally, the highest testosterone levels occur in the early morning hours; therefore, doctors will 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 68 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. A normal total testosterone reading may not necessarily indicate that a man has normal levels of free testosterone. 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.
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- Because low testosterone levels may affect bone mass, the doctor may want to assess any bone loss with 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, he/she may want to examine the gland to see if a tumor is present. Two examination procedures are most common, and neither penetrates the skin. A computed tomography, or CT, is a computer-assisted X-ray process. Magnetic resonance imaging, or MRI, uses a combination of radio waves, high intensity magnetic fields and computer technology to produce images of the body's interior.
Source: The Hormone Foundation - The Public Education Affiliate of the Endocrine Society