and reported transient rejuvenation. In 1927, Fred Koch, using 40 pounds of bovine testicles, extracted 20 mg of a substance, which re-masculinized castrated roosters, pigs, and rats. This testicular hormone substance, which was isolated and synthetized in 1935, was aptly named Testosterone.

Testosterone, which is also produced by the ovaries and adrenal glands, promotes sexual development, increases bone and muscle mass, prevents osteoporosis, and is essential for health and well-being. Males produce twenty times more testosterone than females and their blood levels are 7 times higher, but females are more sensitive to the effects of testosterone, which promote pubic and axillary hair, clitoral enlargement, sexual desire, sexual function, and post-orgasmic relaxation.

In men, testosterone also acts as a health hormone, promoting cardiovascular health, reducing body fat, blood sugar, and blood pressure, increasing well-being and energy, and delaying the frailty of age. Higher testosterone levels increase romantic competitiveness and its levels decline after men fall in love and father children. Moreover, men who maintain higher levels have more extramarital sex, drink and smoke more, and sustain more personal injuries.

Currently, testosterone replacement therapy is fashionable and widely prescribed for both men and women. For women, it is mainly given to improve libido. However, other uses include improving bone and muscle mass, fatigue, and certain types of depression. Potential side effects in women include acne, increased facial hair, decreased scalp hair, and increased risk in breast and gynecological cancers. The science, concerning when to prescribe testosterone for women and in what dose, is still developing. Current treatment trends, which are based on hormone levels, clinical experience, and female demand may radically change once controlled data become available.

Women who have lost their ovaries, are on the birth control pill, or are going through menopause, tend to have lower testosterone levels, but many continue to have healthy libidos. Treating these women based on blood levels alone is not supported by science. Treating women, who are suffering from significant anxiety and depression with hormones, based on their blood or saliva levels, is also unsupported by science even though it is widely practiced.

In contrast, current treatment in men is primarily based on blood testosterone levels, but levels are only measured if there are compelling reasons such as poor libido, osteoporosis, unexplained fatigue, and age-related muscle wasting with increased abdominal fat. Inappropriate use of testosterone, which is also widely practiced, may increase blood volume, worsen sleep apnea, enhance baldness, cause testicular atrophy, and reduce sperm counts. In the US, testosterone prescriptions have increased 500% between 1993 and 2000, and that pattern continues today, fueled by the numerous advertisements advocating the use of testosterone.

A pivotal study by Finkelstein et al. published on September 12, 2013, in the New England Journal of Medicine has clarified some of our current misconceptions. By looking at both testosterone and estrogen levels in men, they found that the consequences of reduced testicular function are caused by both testosterone and estradiol deficiencies:

a) Abdominal fat accumulation, which begins during mild testosterone deficiency, is mainly caused by the low estrogen levels. The accumulated visceral fat increases resistance to insulin, increases blood lipids, invites diabetes, and increases strokes and heart attacks.

b) Muscle and bone strength begin to decline mainly when testosterone deficiency becomes severe, leading to falls, fractures, and dependency.

c) Poor libido and erectile dysfunction are a consequence of both low testosterone and low estrogen levels.

Therefore, blaming male decline on testosterone deficiency alone is no longer tenable. Estrogens play a key role in regulating body fat, sexual function, and bone metabolism. As both estrogen and testosterone deficiencies produce the decline, both estrogen and testosterone are needed to treat it. Luckily, treating with testosterone alone does provide sufficient estrogen because estradiol is a natural metabolite of testosterone.

Whereas only 20% of men over sixty are testosterone deficient, 50% of men over eighty become deficient. Therefore, measuring both testosterone and estradiol levels might prove more helpful in assessing sexual dysfunction, bone and muscle loss, fat accumulation, and geriatric frailty. However, men should only be treated with testosterone because treating them with estrogen will cause chemical castration.