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Men's sexual physiology:  Male sex hormones

The male sex hormones, or androgens, are essential for normal physical development, fertility, sex drive and normal sexual functioning in men. The two main androgens are testosterone and dihydrotestosterone, which is a more powerful variation of testosterone.

Even though testosterone is produced in your testicles (also known as your testes), its production is under the control of the brain, via a complex cascade of messenger proteins: this is fairly similar in both sexes, though in women the control system works on the ovaries, not the testicles.


The first signal for testosterone production comes from the hypothalamus, a structure deep inside the brain which regulates many body functions and keeps our homeostatic balance. The hypothalamus releases a hormone called gonadotropin releasing hormone (GnRH). This affects another structure close by in the brain, called the pituitary gland, which is the main hormone producing gland in the brain; its secretions help to coordinate reproduction and body growth. In response to GnRH the pituitary releases two hormones called luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Both hormones are released into the blood stream from where they reach all parts of the body, including the testicles.

LH stimulates cells called Leydig cells in the testicles to produce testosterone, a fat-based hormone synthesized from cholesterol. Testosterone is released from the Leydig cells and finds its way into the blood stream, which transports it around the body. The cells of tissues which are influenced by testosterone have receptors on their cell walls for the hormone. (An example of this is your muscles, which grow under the influence of the hormone to give you more bulk and strength as a man.) These receptors trigger the activation or deactivation of various genes which then stimulate the production of specific proteins. This means testosterone has a direct influence on anabolism, or protein production, and it's this aspect of their operation which is artificially triggered when men use anabolic steroids to bulk up their muscle mass. Some tissues are able to convert testosterone into its more potent derivative dihydrotestosterone.


The second hormone from the pituitary (FSH) also stimulates cells in the testicles, however this time it's the Sertoli cells in the seminiferous tubules. These cells produce sperm. The last step of sperm production is also aided by testosterone produced by the nearby Leydig cells. 

The androgens have a major impact on the body of a man, an influence which begins before his birth by promoting the development of male reproductive organs and the growth of external male genitalia. During puberty, the production of a boy's testosterone increases, causing the development of male characteristics: growth of his penis and testicles, more muscle tissue, deeper voice, more hair. A higher testosterone level also facilitates a higher sex drive, sperm production, and ejaculation. Additionally, testosterone stimulates the productions of protein all over the body; it is an anabolic hormones - one that stimulates protein production.


Before I say a bit more about artificial anabolic hormones, it's important to describe the regulation of testosterone levels in the blood stream to really understand the consequences of increasing your anabolic hormone levels. Testosterone is regulated by a negative feedback loop in the brain which works like this: high levels of testosterone in the blood directly impact on receptors in the hypothalamus. This results in the hypothalamus releasing less GnRH, which in turn results in less stimulation of the pituitary gland and less production of LH. And less LH results in less production of testosterone by the Leydig cells in your testicles.

Most of you will probably have heard of the illegal use of anabolic steroids by athletes and the wider male population. These anabolic steroids are similar to testosterone and have the same effect of boosting protein production, which means more muscle and more power. However, very large doses need to be introduced into the body from the outside to produce these effects. Such high doses can have damaging effects on various body systems and may produce side effects including liver cancer, kidney damage, wide mood swings and higher levels of aggressiveness, stunted growth, and a higher risk of heart disease. Also, having high levels of testosterone-like hormones in the blood means that the body's own negative feedback loop is activated. The body starts to produce less and less of its own testosterone, which can result in the testicles shrinking, infertility and baldness. 


Testosterone is the most important protein in the male reproductive system. Putting it simply, testosterone is what makes you a man physiologically. And not surprisingly, a fall in testosterone production, for example due to age (andropause), illness or lifestyle choices (such as drinking too much alcohol), has widespread consequences for men. In some men the uptake of testosterone may be inhibited by problems in the testosterone receptors. If you are experiencing sexual problems, loss of sex drive and generally a lack of energy you may want to go to your doctor to get your testosterone levels checked. Replacement therapies are now available, but are expensive and sometimes difficult to get. See the page on low sex drive for men for more information.


Source for the facts cited in this page: 

ABC of Sexual Health (2005) Second edition edited by John M Tomlinson, British Medical Journal Books and Blackwell Publishing.

Principles of anatomy and physiology (2000) Ninth edition by Gerard J. Tortora and Sandra Reynolds Grabowski. Publishers: John Wiley and Sons


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