Testosterone is a member of the steroid family of hormones. Interestingly, this family is derived through chemical conversion from the steroid ring-structure of cholesterol. (You probably thought that cholesterol was bad for you, and the lower the blood level the better, but in reality cholesterol is one of the most important molecules in your body, not the least because it is a precursor to these important hormones.) In fact, there are only minor adjustments to this ring structure (additions or deletions of side chains or electrons) that differentiate molecules with such profoundly different effects as estrogen, progesterone, DHEA, cortisone, and androstenedione, to name a few of the more familiar ones. They are small molecules, which becomes important when we look at what routes of administration are available.
Ninety five percent of circulating testosterone is produced in the testicles under the influence of a hormone released from the pituitary gland called luteinizing hormone (LH). Throughout the day, LH is released in spurts that stimulate the release of testosterone from the testicles. The signals are particularly strong in the early morning which accounts for the higher testosterone levels and thence the spontaneous morning erections and increased libido. By the late evening the levels of testosterone can fall by 50 percent, which then signals the pituitary to rev up its production of LH to start the cycle all over again.
When discussing testosterone levels, we must be sure to distinguish between the total and free fractions found in the blood. The vast majority (between 97 and 99 percent) of testosterone secreted in to the bloodstream is attached (scientists use the term ‘bound’) to a protein ingeniously called ‘sex hormone binding globulin’ (SHBG). It serves to keep the testosterone from being too quickly removed by the kidneys into the urine, thereby maintaining a steady supply for the tissues. The free testosterone is the biologically more important fraction because only it is able to diffuse into the target tissues and alter physiological function.
In addition to the testicular production, which accounts for the vast majority of circulating testosterone, a smaller amount can be created by the conversion of precursor steroid hormones such as androstenedione and androstenediol. These hormones achieved notoriety when home run king Mark McGwire admitted to using them; you can buy them over the counter under such names as Andro-this or that. Unfortunately for McGwire, a number of recent studies have demonstrated that a greater portion of the androstenedione gets converted into estrogens rather than testosterone and therefore there is not a sustained muscle enhancing effect. The estrogens, in contrast, stay elevated for a longer period of time and can cause gynecomastia—enlargement of the male breast tissue. Far from causing an increase in muscle and masculine features, supplementing with these hormone formulations often results in feminization of the user. McGwire has since discontinued using them, and we can assure you that neither his muscularity nor his awesome hitting power derives from his previous use.
Before we move on, we want to clarify some misconceptions about what testosterone is and is not. While testosterone is a member of the group of compounds known as ‘anabolic steroids,’ the muscle and bone building molecules, it is different from the kind this term often denotes—the kind that have been abused by body builders and professional athletes. These include decadurabolin, oxandrolone, and methyltestosterone which are different from testosterone in their molecular structure and not normally found in the human body. These are potent anabolic hormones, but they can have adverse effects on other organ systems, such as the brain and liver, because of this changed structure; therefore, while they have similar muscle building effects, the side effect profiles are not comparable. Because unscrupulous doctors and black marketers sold these drugs in high doses to young men and professional athletes, they—and testosterone along with them—became regulated as schedule III substances like morphine and other narcotics. This has tarnished testosterone’s image amongst doctors and the public to the detriment of many who would benefit from responsible well-monitored TRT.
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