Testosterone is a male hormone that plays a role in the sexual development and characteristics of men. Testosterone is secreted by the testicles and adrenal glands. It is responsible for building muscle mass and bone density, fat distribution, production of hemoglobin, cholesterol metabolism, sexual functioning, concentration, memory and mood.
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Types of Testosterone
There are several types of testosterone in the body. Testosterone either circulates freely in the blood or is bound to several different proteins. According to a 2006 study in Clinical Chemistry, 44 percent of testosterone is bound to sex hormone-binding globulin, or SHBG, 50 percent to albumin, 3.5 percent to cortisol-binding globulin and 2 to 3 percent is free or unbound. Testosterone that is free or bound to albumin is considered biologically active, since testosterone is bound weakly to albumin and can easily become unbound. The sum of free testosterone and albumin bound testosterone is known as bioavailable testosterone. This is the portion of testosterone that is able to enter the cells. Testosterone that is bound to SHBG is biologically inactive because the bond between testosterone and SHBG is strong, and testosterone cannot become unbound from it.
Normal Levels of Testosterone
According to Quest Diagnostics, the normal range of total testosterone levels is 250 to 1,100 nanograms per deciliter, or ng/dL, in men age 18 to 69 and 90 to 890 ng/dL in men 70 to 89. The normal range for free testosterone in men age 18 to 69 is 46 to 224 ng/dL and men 70 to 89 is 6 to 73 ng/dL. Bioavailable testosterone should measure 110 to 575 ng/dL in men age 18 to 69 and 15 to 150 ng/dL in men 70 to 89.
Factors Affecting Testosterone Levels
Testosterone levels vary according to age and stage of sexual development. Levels in men diminish slowly with age beginning at age 30, according to the Cleveland Clinic. Testosterone levels also vary throughout the day, so more than one test may need to be required to get an accurate reading. Testosterone levels tend to be highest in the morning, so most doctors prefer to do testing early in the morning.
Meaning of Results
Low levels of testosterone in males may be caused by illness, late puberty, testicular failure, hypogonadism, genetic diseases, physical injury to the testes or alcoholism. High levels of testosterone may be caused by tumors of the testicles, adrenal gland tumors, anabolic steroids, precocious puberty, congenital adrenal hyperplasia or hyperthyroidism.
Certain medical conditions can change the level of SHBG and albumin in the body. Obesity, steroids use, kidney disease and hypothyroidism may decrease SHBG levels. Conditions that may increase SHBG levels include hyperthyroidism, estrogen use and cirrhosis. This may alter the level of total testosterone, while the free and bioavailable testosterone levels are normal.