Testosterone is typically thought of as a male hormone though it’s produced by both men and women. In men, who produce much larger amounts of the hormone, it leads to development of the uniquely male physical and sexual characteristics, including behavioral changes. In both genders, testosterone is associated with sex drive. Like all hormones, testosterone is secreted directly into the bloodstream. Its effects are determined not by where it is distributed, since it travels throughout the body, but by which tissues have testosterone receptors. These tissues, called testosterone’s target organs, are able to respond to the presence of the hormone in the blood.
Male fetuses begin producing testosterone quite early in life. Receptors on the tissues of the sex organs cause the early ducts, which are the same in both male and female embryos, to begin developing into uniquely male structures. The nub of flesh that would become a clitoris in the absence of testosterone is induced to become the glans of the penis under the hormone’s influence. What would have otherwise become labia fuse to become the scrotum, and the gonads—now testes—descend to fill it. In her book, “Human Physiology,” Dr. Lauralee Sherwood notes that these early gender-differentiating events take place around the third month of gestation.
Bones and Muscles
Testosterone is secreted in small amounts during childhood but again begins to direct development around the onset of puberty. At this time, boy’s testes start producing much larger quantities of the hormone, which leads to further differentiation between male and female body shapes and features. The bones are an important target organ for testosterone—the hormone causes growing bone ends to fuse and thickens the bodies of the bones, leading to increased male bone density as compared to female. Dr. Gary Thibodeau, in his text “Anatomy and Physiology,” notes that boys who secrete high levels of testosterone early in puberty generally don’t reach the same height as boys who secrete testosterone in lower amounts or later in puberty, because the high levels of testosterone can prematurely stop bone growth. Muscles, too, are targets for testosterone. For this reason, pubescent males increase muscle mass dramatically, quickly achieving more extensive musculature than possible for either boys or women.
Secondary Sex Characteristic Tissues
While the secondary male sex characteristics, including deepened voice, extensive body hair and increased sweating are not required for reproduction or sexual function, they nevertheless serve as signals to other humans to help identify males quickly and easily. The tissues associated with these secondary characteristics, including the vocal cords and sebaceous glands of the skin, are also targets for testosterone and respond to its influence.
The relationship between the brain and testosterone is complex. An endocrine gland in the brain, called the anterior pituitary, is responsible for secreting a hormone that increases testosterone production. In addition, the brain itself is affected by testosterone, and the binding of the hormone to receptors in the brain results in behavioral changes, including increased sex drive and increased aggression. Dr. Thibodeau notes that one of the reasons for the behavior changes associated with steroid use is that the brain responds to steroids—essentially synthetic forms of testosterone—just as muscles do. Where muscles enlarge in the presence of anabolic steroids, the brain interprets the extra hormones as a signal to increase aggressive behavior.