Testosterone is a member of the steroid family of hormones called androgens. More and more, this family is derived through chemical conversion from the steroid ring-structure of cholesterol. This hormone plays a vital role in the bodies of both men and women.

There are only minor differences between this hormone and others such as estrogen, progesterone, DHEA, cortisone, and androstenedione. They are small molecules, which becomes important when we look at what routes of administration are available.

As commonly known, testosterone stimulates the development of the penis and testes, growth of facial and pubic hair, deepening of the voice, changes in body shape, growth of bones, and increased muscle mass and strength in males. It helps maintain sex drive and the production of sperm cells, and it may play a role in balding. Mood is also affected by testosterone, and low levels of the hormone can cause severe and prolonged depression as well as fatigue, to name a few conditions. High levels of testosterone appear to promote good health in men, for example, lowering the risks of high blood pressure and heart attacks. High testosterone levels also correlate with risky behavior, however, including increased aggressiveness and sometimes even smoking, which may cancel out these useful health benefits.

Ninety-five percent of circulating testosterone is produced in the male testicles under the influence of a hormone released from the pituitary gland called luteinizing hormone (LH). Throughout the day, LH is released in spurts that stimulate the release of testosterone from the testicles. The signals are particularly strong in the early morning, which accounts for the higher testosterone levels and spontaneous morning erections and increased libido. By the late evening the levels of testosterone can fall by 50-percent, which signals the pituitary gland to increase its production of LH to restart the cycle.