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In recent years, researchers (and pharmaceutical companies) have focused on the effects of testosterone deficiency, especially among men. In fact, as men age, testosterone levels drop very gradually, about 1% to 2% each year — unlike the relatively rapid drop in estrogen that causes menopause. The testes produces less testosterone, there are fewer signals from the pituitary telling the testes to make testosterone, and a protein (called sex hormone binding globulin (SHBG) increases with age. All of this reduces the active (free) form of testosterone in the body. More than a third of men over age 45 may have reduced levels of testosterone than might be considered normal (though, as mentioned, defining optimal levels of testosterone is tricky and somewhat controversial).

Symptoms of testosterone deficiency in adult men include:

• Reduced body and facial hair
• Loss of muscle mass
• Low libido, impotence, small testicles, reduced sperm count and infertility
• Increased breast size
• Hot flashes
• Irritability, poor concentration and depression
• Loss of body hair
• Brittle bones and an increased risk of fracture

Some men who have a testosterone deficiency have symptoms or conditions related to their low testosterone that will improve when they take testosterone replacement. For example, a man with osteoporosis and low testosterone can increase bone strength and reduce his fracture risk with testosterone replacement.

As surprising as it may be, women can also be bothered by symptoms of testosterone deficiency. For example, disease in the pituitary gland may lead to reduced testosterone production from the adrenal glands disease. They may experience low libido, reduced bone strength, poor concentration or depression.

Currently, testosterone therapy is approved primarily for the treatment of delayed male puberty, low production of testosterone (whether due to failure of the testes, pituitary or hypothalamus function) and certain inoperable female breast cancers.

However, it is quite possible that testosterone treatment can improve symptoms in men with significantly low levels of active (free) testosterone, such as:

• Generalized weakness
• Low energy
• Disabling frailty
• Depression
• Problems with sexual function
• Problems with cognition.

However, many men with normal testosterone levels have similar symptoms so a direct connection between testosterone levels and symptoms is not always clear. As a result, there is some controversy about which men should be treated with supplemental testosterone.

Testosterone therapy may make sense for women who have low testosterone levels and symptoms that might be due to testosterone deficiency. (It's not clear if low levels without symptoms are meaningful; treatment risks may outweigh benefits.) However, the wisdom and effectiveness of testosterone treatment to improve sexual function or cognitive function among postmenopausal women is unclear.

People with normal testosterone levels are sometimes treated with testosterone at the recommendation of their doctors or they obtain the medication on their own. Some have recommended it as a "remedy" for aging. For example, a study from Harvard Medical School in 2003 found that even among men who started out with normal testosterone results noted loss of fat, increased muscle mass, better mood, and less anxiety when receiving testosterone therapy. Similar observations have been noted among women. However, the risks and side effects of taking testosterone when the body is already making enough still discourages widespread use.

Testosterone is so much more than its reputation would suggest. Men and women need the proper amount of testosterone to develop and function normally. However, the optimal amount of testosterone is far from clear.

Checking testosterone levels is as easy as having a blood test. The difficult part is interpreting the result. Levels vary over the course of the day. A single low level may be meaningless in the absence of symptoms, especially if it was normal at another time. We need more research to know when to measure testosterone, how best to respond to the results and when it's worthwhile to accept the risks of treatment.