Sexual Function and Libido
Abundant clinical literature shows that testosterone levels directly and indirectly influence the fundamental components of male sexual function, including genital development, sex drive, ejaculation, penile sensitivity, and erectile function.
Male potency and libido are interdependent factors, and evidence suggests that there is a consistent, graded relationship between testosterone levels and sexual interest, particularly in men with suboptimal amounts. Researchers reported a clear relationship between testosterone levels during puberty and sexual timing and activity, with higher levels of salivary testosterone associated with a four-fold increase in sexual activity. A review of recent studies on testosterone therapy notes that testosterone boosts, in a dose-response fashion, both erectile function and sexual libido. This may be one reason why testosterone is among the most commonly used therapies for impotence in men. Experts advise, however, that this approach is best utilized only after an initial assessment of bioavailable levels of testosterone to establish clinical need. 13
With advancing age, testicular function often declines and levels of bioavailable testosterone plummet, corresponding to a marked decrease in sexual function. By the time a man reaches age 50, his bioavailable levels of testosterone may be half their youthful levels.14 Significantly, testosterone loss has been shown to precede — not simply accompany — both a drop in sexual interest and frequency of erections. 15
Other health conditions commonly associated with impotence in men, such as diabetes, vascular dysfunction, thyroid imbalances, arthritides, and various systemic diseases, display powerful synergistic effects upon testosterone levels.16-21 A positive correlation has also been established between testosterone levels and male fertility, with testosterone loss corresponding to oligospermic and azoospermic conditions.
Musculoskeletal Health
As an anabolic hormone, testosterone provides the important hormonal source for bone and muscle tissue growth throughout the body.
Bone Growth and Remodeling
The impact of osteoporosis in men remains a severely neglected area of health care. The National Osteoporosis Foundation reports that men over 50 have a greater chance of suffering an osteoporosis-related fracture than they do of developing prostate cancer.22 In fact, nearly one out of every three hip fractures occurs in men — and the resulting mortality — one-third of those men will die within the same year — is higher than it is in women.
Increasing evidence reveals that gonadal function — and specifically the production of testosterone — plays a central role in bone metabolism and growth. Surges in testosterone during puberty trigger the accelerated bone growth and density of bones that spurs sexual dimorphism of the human skeleton. As men age, the sex steroids continue to impact strongly the process of bone formation and resorption. Androgens may accomplish this by binding directly to androgen receptors in cells, such as osteoblasts, or by triggering an indirect effect via their aromatization to estrogen. 23
Hypogonadism, which may occur as a result of genetics, environment, aging, or illness, can accelerate bone turnover by increasing the recruitment of osteoclasts from bone marrow. In this way, chronically low levels of testosterone can predispose men to osteoporosis. Jackson and colleagues found that testosterone-deficient men had a five-fold greater likelihood of suffering hip fractures than controls. They called gonadal deficiency “an important and heretofore understudied” factor predisposing men to increased hip fractures, and concluded that “prevention of hip fractures in men may involve early recognition and treatment of testosterone deficiency.”24
Numerous studies consistently show that long-term testosterone replacement therapy to combat hypogonadism improves bone mineral density and reduces bone turnover.
Muscle Mass and Sarcopenia
Testosterone displays a marked ability to build muscle tissue in the body, and may trigger its anabolic effects through a variety of physiological mechanisms. Testosterone increases nitrogen stores in the body, promotes protein synthesis, and raises circulating levels of insulin-like growth factor (IGF-1), all of which are known to enhance muscle formation.25-27
Numerous studies report that testosterone administration increases muscle size, strength, and lean body mass in both young and old hypogonadal men, as well as in older healthy men with low to low-normal testosterone levels. Besides conferring positive benefits on a man’s well-being by ensuring optimal physical strength and stamina, this has profound clinical implications for preventing morbidity and mortality associated with frailty and wasting conditions.
Clinical Studies
Testosterone Effects
Dutch researchers point out that “loss of muscle strength resulting in frailty is the limiting factor for an individual’s chances of living an independent life until death.” Maintaining optimal levels of testosterone provides an important therapeutic tool to ensure mobility, function, and quality of life in men as they age.
Several recent studies have revealed testosterone’s powerful role in preventing wasting conditions associated with AIDS, cancer, lung disease, cancer, and chronic infection. A team of investigators from Massachusetts General Hospital found that treating AIDS patients for testosterone deficiency resulted in a significant increase in lean body mass, which is associated with higher survival rates, as well as improved well-being, appearance and overall quality of life. The authors of this double-blind, placebo-controlled study concluded that androgen testing could be valuable in identifying men with AIDS wasting syndrome who may benefit from testosterone replacement therapy. The percentage of HIV-infected men who are hypogonadal is estimated to range from 20% to 70%.28-29
