The Endocrine System and Testosterone, An Introduction
Testosterone is a steroid hormone seen in mammals, reptiles, birds, and other vertebrates and is one of the most highly conserved molecules in multicellular organisms. This will be an introduction to the chemistry, biology and mechanics of testosterone, a molecule that we’re almost all familiar with, yet don’t quite understand.
Starting broadly, let’s tackle the properties and important information about hormones and the endocrine system in general. The endocrine system is a lot like the nervous system and they both are the body’s two major communication systems. Unlike the nervous system where communication is rapid over short distances (think synapses), the signals sent by the endocrine system may have much longer delays, last for a much longer period of time, and travel much greater distances. While they both use chemicals as their messengers, the endocrine system utilizes the bloodstream as its highway while the nervous system does not (for the most part).
The endocrine system consists of glands that secrete hormones that enter the blood and are carried to the target cells upon which they act. Different hormones have different target cells and one cell may secrete several different types of hormones! Overall, the endocrine system is a major player in your body’s control systems of metabolism and homeostasis.
Steroid hormones are lipid based molecules made mostly of carbon and hydrogen making them nonpolar and have a very low solubility in water (ever wonder why your injectible testosterone is suspended in oil?). Steroid hormones are produced primarily by the adrenal cortex of the kidneys and your reproductive organs (ovaries and testes). In addition to testosterone, cortisol, aldosterone, cholesterol, and estradiol are steroid hormones as well.
All steroid hormones are derived from cholesterol. Cholesterol is stored inside the cell and once the cell is stimulated the free cholesterol is transformed by a myriad of chemical processes in several organelles into different hormones. Therefore steroid hormones and testosterone are made on demand. Once the final product is formed it diffuses (passively - remember the lipid bilayer and the fact that testosterone is a lipid based hormone? Yeah…) across the cell membrane and into the interstitial fluid and later into circulation. Again mentioning the lipid nature of testosterone it is not highly soluble in blood and thus is largely transported in the plasma bound to carrier proteins. *Something to note* Following the release into the blood steroid hormones may undergo further modification - testosterone can be converted into estradiol and vice versa. Thus, the major male and female sex hormones are not unique to males and females however the concentrations of the hormones vary substantially between the two.
Because hormones are transported by the blood they can reach virtually all tissues, which is what we mean when we say that testosterone acts “systemically”. The response to hormones, however, is very specific and involves only the target cells for that hormone. The ability to respond depends on the specific receptor for those hormones, in this case testosterone, on or in the target cell. Testosterone receptors happen to be within the cell and the binding of the hormone to a receptor leads to the activation (or in some cases the inhibition) of the transcription of certain genes - causing a change in the synthesis rate of proteins coded for by those genes.
With hormones already being a sort of broadcast molecule with long-acting and long-distance capabilities - this particular mechanism of hormone action is long even by hormone standards. But as you have probably seen, the effects of steroid hormones and testosterone in particular are profound.
This process is much more complicated than described here, but we hope this diluted version of anatomy and physiology 101 is helpful to some!
Much of this information was adapted from “Human Physiology: The Mechanisms of Body Function” 11th edition by Widmaier, Raff & Strang.