Testosterone is an androgen steroid hormone made by your body and is responsible for the normal growth and development of the male reproductive organs and for maintenance of other sexual characteristics. In men, testosterone is produced primarily in the testes, the reproductive glands that also produce sperm; in women, it is produced in the ovaries. The amount of testosterone produced by the testes is regulated by the hypothalamus and the pituitary gland. It is the principal male sex hormone and an anabolic steroid. In addition, testosterone is essential for health and well-being as well as the prevention of osteoporosis.

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

The physiological actions of testosterone in males are far reaching. For example:

  • Growth and maturation of the penis, scrotum and testes during puberty
  • Enlargement of the larynx (voice box) that results in a deepening of the voice
  • Stimulation of hair growth – especially in the pubic area, chest, face, and sometimes the back
  • Increases in libido (sex drive)
  • Increases in basal (resting) metabolic rate
  • Increases in number of red blood cells and total blood volume
  • Increases in muscle protein synthesis resulting in increased muscle mass
  • Increased bone strength due to calcium retention
  • Reductions in muscle glycogen breakdown during exercise
  • Elevated mood and energy levels
  • Increases in skin thickness and darkness
  • Promotion of sodium and water retention in the kidneys
  • Decreased growth of hair on top of the head
  • Increased activity of the sweat glands, sometimes resulting in acne