(MayoClinic.com) The possibilities of testosterone therapy are enticing — increase your muscle mass, sharpen your memory and concentration, boost your libido, and improve your energy level. As you get older, testosterone therapy may sound like the ultimate anti-aging formula. Yet the health benefits of testosterone therapy for age-related declines in testosterone aren't as clear as it may seem. Find out what's known — and not known — about testosterone therapy for normal aging.

Testosterone is a hormone produced primarily in the testes. For men, testosterone helps maintain:

• Bone density
• Fat distribution
• Muscle strength and mass
• Red blood cell production
• Sex drive
• Sperm production

If you have an unusually low level of testosterone (hypogonadism), your doctor may prescribe a synthetic version of testosterone. You may be able to choose from testosterone injections, patches or gels.

Testosterone peaks during adolescence and early adulthood. As you get older, your testosterone level gradually declines — typically about 1 percent a year after age 30.

Some men have a lower than normal testosterone level without signs or symptoms. For others, low testosterone may cause:

• Changes in sexual function. This may include reduced sexual desire, fewer spontaneous erections — such as during sleep — and infertility.
• Changes in sleep patterns. Sometimes low testosterone causes insomnia or other sleep disturbances.
• Physical changes. Various physical changes are possible, including increased body fat, reduced muscle bulk and strength, and decreased bone density. Swollen or tender breasts (gynecomastia) and hair loss are possible. You may experience hot flashes and have less energy than you used to.
• Emotional changes. Low testosterone may contribute to a decrease in motivation or self-confidence. You may feel sad or depressed, or have trouble concentrating or remembering things.

It's important to note that some of these signs and symptoms are a normal part of aging. Others can be caused by various underlying factors, including medication side effects, thyroid problems, depression and excessive alcohol use. A blood test is the only way to diagnose a low testosterone level.

Testosterone therapy can help reverse the effects of hypogonadism, but it's unclear whether testosterone therapy would have any benefit for older men who are otherwise healthy. Although some men believe that taking testosterone medications may help them feel younger and more vigorous as they age, few rigorous studies have examined testosterone therapy in men who have healthy testosterone levels — and some small studies have revealed mixed results. For example, in one study healthy men who took testosterone medications increased muscle mass but didn't gain strength.

Testosterone therapy has various risks. For example, testosterone therapy may:

• Contribute to sleep apnea — a potentially serious sleep disorder in which breathing repeatedly stops and starts
• Cause your body to make too many red blood cells (polycythemia), which can increase the risk of heart disease
• Cause acne or other skin reactions
• Stimulate noncancerous growth of the prostate (benign prostatic hyperplasia) and possibly stimulate growth of existing prostate cancer
• Enlarge breasts
• Limit sperm production or cause testicle shrinkage

If you wonder whether testosterone therapy might be right for you, work with your doctor to weigh the risks and benefits. A medical condition that leads to an unusual decline in testosterone may be a reason to take supplemental testosterone. However, treating normal aging with testosterone therapy remains controversial.

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