Testosterone is the male sex hormone. Testosterone is a steroid hormone, with a chemical structure closely resembling that of the anabolic steroids used to produce increases of mass and strength among athletes. Testosterone is chemically classified as an androgen, one of the group of hormones that promote the growth and development of the male body characteristics, including greater muscle mass. The contrasting female growth hormones are estrogens, chemically similar to the male hormone. Because testosterone promotes the growth of male characteristics, it has been long desired as a muscle and strength building agent. While testosterone is essential in the creation of the physical distinctions between the male and female structures, the female endocrine system also produces testosterone, though in much lesser quantities than males.

Like all steroids, testosterone is constructed from four carbon rings; it is the location of various oxygen and hydrogen molecules within the ring structure that distinguishes testosterone from other well-known steroids used by athletes, including stanozolol, dianabol, and nandrolone.

Testosterone formation begins within the body as a process-utilizing cholesterol, itself a byproduct of the fats ingested through diet and absorbed for storage within the body as triglycerides. Within the body, cholesterol is used to form testosterone and numerous other hormones. As a hormone, testosterone is a product of the body's endocrine system, a sophisticated series of glands that are subordinate to the functions of the thyroid gland, which is itself directed in its actions by the region of the brain known as the hypothalamus. The hypothalamus/pituitary gland/testes glandular relationship is referred to as the gonadal axis. The testes produce between 4 mg and 7 mg of testosterone each day in a healthy male.

Hormones function as chemical signals directed by the brain to compel bodily organs or systems to function in a particular way. The release of adrenaline when a threat to the body is perceived and the production of the growth hormone during the period of adolescence are two common examples of hormone secretion and function. With testosterone, any disruption of the signals delivered along the gonadal axis will interfere with testosterone production.

Within the endocrine system, the primary source of testosterone production is the testes, the pair of male glands located inside the scrotum. The adrenal glands, positioned above each kidney, are a secondary source of this hormone. Testosterone is of fundamental importance to human function in a number of areas. Testosterone influences the development of all primary and secondary sexual characteristics in males, including their sexual function, and appearance attributes such as voice characteristics and the growth of body hair. The general speed and quality of male tissue growth is influenced by testosterone, as is the overall development and maintenance of muscle mass and strength. Adequate levels of testosterone production are necessary for the formation and preservation of bone structure and bone density. Testosterone contributes to effective brain activity, including learning and memory skills. Finally, testosterone contributes to the maintenance of the body's general energy levels necessary for effective function in all physical activities.

For males over 50 years of age, there is a natural decline in the amount of testosterone produced by the testes. This decline can present a significant difficulty for older male athletes, as reduced testosterone production will contribute to muscle weakness, a potential decrease in sexual function, as well as the decreased bone density that typically contributes to osteoporosis, the bone-thinning disease. The most widely used treatments for testosterone deficiency involve a hormone replacement therapy, which provides replacement testosterone to the body either through a transdermal (skin-applied) patch, or intramuscular injection.

In international athletic competition governed by the World Anti-Doping Agency (WADA), testosterone is specified as a banned substance on the Prohibited List of all illegal performance-enhancing substances. Testosterone is classed as an anabolic androgenic steroid, an illegal steroid that is intended to produce or facilitate male growth and physical characteristics in an athlete. Prior to the ascendancy of WADA in the late 1990s as the foremost drug regulatory agency in athletics, a number of notable world-class athletes had been the subject of positive testosterone tests, among them American sprinter Dennis Mitchell, American middle distance runner Mary Decker Slaney, and Dutch shot putter Erik de Bruin. Unlike other chemically produced performance-enhancing substances, testosterone is present in the body of all athletes in varying degrees. Testosterone testing is based on whether the hormone appears to be present in an unnatural amount, beyond the range that would typically be expected in that person. The WADA standard for the testosterone range is generally where the amount of testosterone present through testing is greater than 4:1 ratio to the expected levels; at those levels, a positive drug test is deemed to have resulted.

WADA have developed similar testing standards for all potentially endogenous substances (like testosterone, those capable of originating within the body), as opposed to the stricter limits defined for exogenous substances (those that can only originate outside of the body, such as most anabolic steroids and stimulants).

SEE ALSO Anabolic steroids; HGH; Hormones; Nandrolone.