Testosterone.

http://en.wikipedia.org/wiki/Testosterone

Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles,birds, and other vertebrates. In mammals, testosterone is secreted primarily by the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid.

On average, in adult males, levels of testosterone are about 7–8 times as great as in adult females,but, as the metabolic consumption of testosterone in males is greater, the daily production is about 20 times greater in men.Females are also more sensitive to the hormone.

Health effects

In general, androgens promote protein synthesis and growth of those tissues with androgen receptors. 

• Anabolic effects include growth of muscle mass and strength, increased bone density and strength, and stimulation of linear growth and bone maturation.

Pubertal effects

• Increased muscle strength and mass,Deepening of voice,
• Shoulders become broader and rib cage expands
• Enlargement of sebaceous glands. This might cause acne.

Aggression

The "evolutionary neuroandrogenic theory" focuses on the hormone testosterone as a factor influencing aggression and being evolutionarily beneficial during certain forms of competition. In most species, males are more aggressive than females. 

Athletics

Testosterone can be used by an athlete in order to improve performance, but it is considered to be a form of doping in most sports. There are several application methods for testosterone, including intramuscular injections, transdermal gels and patches, and implantable pellets. Supplement of the hormone results in lower metabolic production via the Farquharson phenomenon, creating long-term dependence for improved performance level.

Anabolic steroids (including testosterone) have also been taken to enhance muscle development, strength, or endurance. They do so directly by increasing the muscles' protein synthesis. As a result, muscle fibers become larger and repair faster than the average person's. After a series of scandals and publicity in the 1980s (such as Ben Johnson's improved performance at the 1988 Summer Olympics), prohibitions of anabolic steroid use were renewed or strengthened by many sports organizations. Testosterone and other anabolic steroids were designated a "controlled substance" by the United States Congress in 1990, with the Anabolic Steroid Control Act.The use is seen as being a seriously problematic issue in modern sport, particularly given the lengths to which athletes and professional laboratories go to in trying to conceal such abuse from sports regulators. 

Some female athletes may have naturally higher levels of testosterone than others, and may be asked to consent to a “therapeutic proposal”, either surgery or drugs, to decrease testosteone levels to a level thought acceptable to compete fairly with others.

Hypothalamic–pituitary–testicular axis