In 1849, Berthold, a German physiologist and zoologist, named Arnold Berthold conducted what is thought to be the first study of behavioral endocrinology. Berthold noted that male chickens or roosters,  had distinct differences that separated them from their female counterparts. He noticed that the roosters had a higher sex-drive, a distinct waddle, a more profound comb, crowed more often, and had more muscle. He deduced that there must be some “unseen” source leading to these observable differences. After a couple dissections of both genders, it might not take too long to figure out that this unseen source, was indeed the testes.

He removed the testes from some roosters and noticed that they did not develop the characteristics of roosters which still contained their testes. The castrated roosters did not crow as often, did not exhibit aggressive behavior, had no comb, had less muscle, and had no desire to copulate. In contrast, some of those same capons (castrated roosters) that were implanted with testes (those that actually reestablished a blood connection), began developing those characteristics that were beforehand obsolete. His findings proved that something in the testicles lead to masculiniziation.

Following suit about 40 years later, a Harvard professor, Brown-Sequard, made a “rejuvenating elixer” composed of guinea pig and dog testicles. He injected this just under the skin, which led to probably no more than a placebo effect, but he was convinced it gave him more energy. Though short-lived, he began the trend of attempting to inject exogenous substances derived of the testes.

Fast-forward another 40 years, and a from the University of Chicago by the name of Fred Koch gained access to a massive amount of bull testicles by way of the Boston Stock Yard. 40 lbs. of testicles later, he and a grad student extracted roughly 20mg of a testicular “substance” and administered them to various capons, rats, dogs and other castrated animals. Despite not having testicles themselves, these animals remasculinized making the experiment a success. The implications of this experiment were that a substance derived from the testes can be administered to achieve masculinizing effects, which in-turn perpetuating further research.

In the 1930s large scale research studies were done by numerous pharmaceutical companies. Organon group in the Netherlands were the first to isolate the hormone which the discussed in a Paper “On Crystalline Male Hormone from Testicles (Testosterone)”. The hormone was finally given a name: “testosterone”.

Synthesis of the hormone would not be possible for about another year until a couple of European scientists Butenandt and Ruzicka, figured out how to synthesize testosterone from a cholesterol base for which they were jointly awarded the Nobel Prize in 1939. Because the molecular structure was also now identifiable, it became obvious that modifications could be made to the hormone, specifically, esterification and alkylation.