About Testosterone Testosterone is an important hormone produced in men’s bodies. Testosterone stimulates the development of the penis and testes, growth of facial and pubic hair, deepening of the voice, changes in body-shape, and increased muscle mass. It helps maintain sex drive and the production of sperm cells. Mood is also affected by testosterone, and low levels of the hormone can cause depression as well as fatigue. Testosterone is produced primarily in the testes (95%) and the outer part of the adrenal glands called the adrenal cortex (5%). The testes receive chemical signals from the pituitary gland, which is in the brain. The pituitary gland receives signals from another brain-based gland, the hypothalamus. The hypothalamus secretes gonadotropin-releasing hormone (GnRH). This signals the pituitary gland to produce and secrete luteinizing hormone (LH). LH orders the testes to begin producing testosterone. As the testes continue producing testosterone, the body signals the pituitary gland telling it to make less LH. This, in turn, slows down the production of testosterone.  Normally this feedback loop helps to maintain appropriate serum testosterone concentration. There is a range of values that indicates a "normal" blood level of testosterone. Testosterone levels vary from hour to hour, so fluctuations can be seen in men with no apparent problems. However, doctors check to see if a man’s blood testosterone level falls into a generally acceptable range of values [about 300 to 1,100 nanograms per deciliter (ng/dl) depending on the laboratory values]. Generally, the highest testosterone levels occur in the early morning hours, so measurements often are taken at this time. Laboratory determination of low testosterone is a complex process. Testosterone circulates in the blood in three forms: about 60 percent of testosterone is bound tightly to a protein called sex hormone-binding globulin (SHBG); about 38 percent is weakly bound to another protein called albumin; and about two percent circulates freely in the bloodstream. If a doctor suspects that someone is not producing enough testosterone, he/she will perform a simple blood test to check if the total blood testosterone level falls into the generally acceptable range. The doctor also may instruct the laboratory to measure the amount of bound testosterone and the amount of free testosterone in the body.  A low testosterone reading in one of the above areas may not necessarily indicate that a man is suffering from low testosterone. Some men with low levels of SHBG and normal blood levels of free testosterone may have only slightly reduced levels of total testosterone. Free (or unbound) testosterone tests are a good idea for older men. Certain genetic conditions such as Klinefelter’s syndrome, Kallmann’s syndrome and Prader-Willi syndrome can cause lowered testosterone production in boys and young men. In addition, testosterone production can be lowered by injury, inflammation and tumors. Chemotherapy and radiation therapy also may damage testosterone-producing cells. Not only does the amount of testosterone produced decline with age, the morning spike of testosterone seen in young men is not typically found in older men. The pituitary glands of older men also produce less luteinizing hormone (LH), which decreases testosterone production. Aging men may have an increase in SHBG, which can cause a reduction in the amount of freely circulating testosterone that is available to the body. Symptoms related to low testosterone include: decreased sex drive, lowered sperm count, hot flashes, fatigue, trouble concentrating and depression. Men who have a severe and prolonged reduction of testosterone also may experience loss of body hair, reduced muscle mass and decreased bone mass.