Testosterone

Definition

Testosterone is a hormone that is responsible for the development and maintenance of masculine [Page 983]characteristics. Testosterone is released into the bloodstream by the testes (testicles) in males and to a lesser extent, by the adrenal cortex and ovaries in females. Not only does testosterone influence the growth and development of masculine physical characteristics, such as the penis and the beard, but testosterone is also related to masculine psychological characteristics and social behaviors, including aggression, power, sexual behavior, and social dominance. In addition, social experiences such as competition can cause testosterone levels to rise or fall.

Background and History

The history of testosterone research dates back to ancient times, when farmers observed that castrated animals (animals whose testes had been removed) were not very aggressive and had low sex drives. Castrated humans showed similar changes in behavior.

In 1849, German scientist Arnold Berthold conducted the first formal experiment involving testosterone. It was already known that when chickens were castrated during development, they became more docile than normal roosters. These castrated chickens, called capons, did not fight with others and did not show normal mating behavior. But when Berthold implanted testes from other birds into the abdomen of these capons, they developed into normal roosters. Berthold concluded that the testes must influence aggression and sexual behavior by releasing a substance into the bloodstream.

In 1935, Dutch researchers identified this substance, a hormone that they named testosterone. Later that same year, another group of researchers developed a method for producing testosterone from cholesterol. Through the development, in the 1960s, of a method called radioimmunoassay, researchers were able to measure the amount of testosterone circulating in the bloodstream, and shortly after that, a technique was developed to measure testosterone levels in saliva. The ability to measure testosterone levels through saliva rather than blood has made it easier and more practical to conduct research in humans.

Effects

Testosterone exerts its effects during three different life stages: the perinatal period (which includes pregnancy and the period shortly after birth), puberty, and adulthood.