How would someone go about producing synthetic testosterone?
A company that makes fake testosterone buys a plant, usually a soy plant, and works it up by extracting and back-extracting and doing chemistry. They get a fraction and that fraction undergoes two or three steps and — voilà— you have testosterone. That is called semi-synthesis. If you try to make testosterone by getting many, many carbon and oxygen atoms and sewing them together, that would be expensive. So they go to a natural source — the soy plant — and extract it. Then it is only a couple of steps and you have synthesized testosterone. But fortunately for us who do the testing, using the soy plant is going to create a different carbon isotope ratio than natural testosterone. So the carbon isotope ratio will tell us if it is natural or not.
