Testosterone
The first step to understanding anabolic steroids and how they work begins with a thorough understanding of testosterone itself.
Literally every anabolic steroid is a synthetic derivative of testosterone, or a modified derivative of testosterone.
In other words, the chemical structure of testosterone is modified in various ways and these modifications create new anabolic steroids, which are considered testosterone analogs (or derivatives).
In this case, testosterone is the hormone known as the precursor hormone to the derivative.
In other descriptive terms, testosterone is also called an ambiguous hormone or a precursor hormone to a derivative or analog.
Testosterone is produced endogenously by the human body in the Leydig cells of the testes.
The term 'endogenous' means that testosterone is produced within the human body (by the body's own cells), while the term 'exogenous' means that the substance is produced outside the body, for example through ingestion, injection, etc.
Within the human body, there are two more endogenous hormones that are utilized as precursor hormones for the production of anabolic steroid derivatives.
These are Nandrolone and Dihydrotestosterone (DHT).
Testosterone is metabolized in the body to dihydrotestosterone via the 5-alpha reductase (5AR) enzyme (i.e., dihydrotestosterone is a metabolite of testosterone),
Nandrolone is produced as a byproduct of the aromatization (conversion) of testosterone to estrogen [1].
Given these facts, it's no surprise that testosterone is literally the origin of all anabolic steroids.
Without testosterone, DHT and nandrolone would not exist, and therefore, if DHT and nandrolone did not exist, their individual derivatives and analogs would not exist either.
Testosterone is the primary sex hormone in men.
Hormones are defined and classified as chemical messengers in the human body, which means that they send messages to different cells and tissues in the body to tell them what to do (grow muscle tissue, heal and repair, produce important components, perform specific tasks, etc.
Without different types of hormones, all functions within the body would be unregulated and out of control.
The average man's testosterone production depends on many factors, including an individual's genetics, age, lifestyle, nutritional habits, and activity level.
On average, the median testosterone production in men has been found to be between 50 and 70 mg per week.
The number an individual may fall within this range depends on the aforementioned factors.
It is known that the most pronounced effects of the hormone testosterone are seen and experienced during puberty, evidenced by an increase in testosterone production and secretion, which typically reaches its highest endogenous level in a man's lifetime at this point.
This significant increase in testosterone is responsible for a number of very important physiological changes in the male human body.
Testosterone, like almost all hormones, governs a wide variety of functions within the body.
The nature of hormones is to regulate systemic functions throughout the body, and testosterone is no exception.
The importance of testosterone in the body and its functions
As the primary sex hormone in men, testosterone is responsible for the development and maintenance of male secondary sex characteristics (the development and growth of the male sex organs, including the voice, growth of body and facial hair, increased sebum secretion from the skin, spermatogenesis (the development of sperm), and increased libido and sexual performance).
All of these functions are known as male secondary sex characteristics and androgenic (masculinizing) effects, and cannot function properly or develop efficiently in the presence of inadequate testosterone levels.
While the effects of promoting muscle growth are also considered androgenic, the effects of promoting muscle growth are more independently categorized as anabolic.
The word 'anabolic' means to promote the growth of tissue within the body, in this case, the growth of muscle tissue.
This occurs through testosterone's ability to signal an increase in the rate of protein synthesis (the rate at which the body can synthesize and create new strands of contractile proteins within muscle tissue).
This is why, on average, men have more muscle mass than women, and why men generally weigh more than women.
Women have very little testosterone, which is evidenced by the fact that the average man has been found to endogenously produce about 2.5 to 11 mg of testosterone daily [2].
In comparison, women produce about 0.25 mg of testosterone each day, which is about 90% (or one-tenth) less than men.
In women, the main sex hormone is estrogen, which is also a steroid hormone, although not an anabolic steroid.
Because of this difference, estrogen has very different effects in the body compared to testosterone, resulting in significant differences between men and women.
For example, women naturally have a "softer" tone and store more body fat than men, which is typical because estrogen promotes fat retention/storage in various key areas of the body that are important for female-specific roles such as pregnancy and fetal development.
In addition to these differences, women tend to be shorter than men, have much less muscle mass, and show age-related bone deterioration much more readily.
This is a direct result of the differences in hormonal dynamics between men and women.
How testosterone works specifically at the cellular level
As with all hormones, the systemic and cellular effects of testosterone are very complex, involving a variety of mechanisms that include both direct and indirect effects.
All anabolic steroids are essentially derivatives of testosterone, so just as a son shares genetic traits inherited from his parents, all anabolic steroids share these traits.
There are many different tissues in which testosterone exerts its effects.
Of course, the action of testosterone begins when it is pumped into the body and transported throughout the body via the bloodstream. This movement and transportation pathway allows the hormone to freely travel to various target tissues within the body, where it acts as a messenger telling the cells within those tissues what to do.
Specific target tissues for testosterone include muscle tissue (skeletal muscle), subcutaneous and dermal tissue (skin and under the skin, respectively), scalp, kidneys, bones, central nervous system, and prostate. What happens in these tissues is the same as the general actions and activities of all hormones.
Hormones bind to receptors located on or within the cells of a particular tissue type and deliver a message to the cell that tells the cell to perform a specific task.
In the case of steroid hormones like testosterone and estrogen, specific receptors are located inside the cell.
Testosterone specifically binds to the androgen receptor there to initiate its effects.
Because the steroidal nature of testosterone, estrogen, cortisol, or any other type of steroid hormone allows the hormone to be fat-soluble, only steroid hormones have the ability to bind to receptors located inside the cell.
This means that the hormone can easily pass through the phospholipid (fat) bilayer of the cell membrane. Peptide (protein) hormones cannot do this.
Other types of hormones, such as peptide hormones (also known as protein hormones), cannot travel inside the cell and interact with receptors on the cell membrane, so they must bind to receptors located on the outer surface of the cell membrane.
Thus, testosterone can only affect certain tissues and cells in the body because they possess the specific hormone receptors (androgen receptors) it needs.
All three types of hormones (steroid hormones, peptide hormones, and monoamine hormones) work through these hormone-receptor interactions, and that's what all hormones do.
It's a very vague and non-specific description, but the interaction of a hormone binding to a receptor site is described in science and biology as a key and a lock: the key is the hormone, the lock is the receptor, and both must fit together almost perfectly for a specific action to occur.
Non-steroidal hormones, such as peptide and monoamine hormones, work in the same lock and key fashion, but as mentioned earlier, they bind to and activate receptors located on the outer surface of cells.
The way non-steroidal hormones transmit signals through receptors is different from steroidal hormones, where peptide or monoamine hormones bind to receptors located on the cell surface, allowing various enzymes and proteins within the cell to act as messengers.
These messenger-activated proteins are known as ATP (adenosine triphosphate) and cAMP (cyclic AMP), and they travel within the cell to the nucleus of the cell to activate gene transcription.
While the general function of non-steroidal hormones is the same as steroidal hormones, their actual steps and specific actions at certain stages are actually different.
As mentioned earlier, testosterone diffuses through the cell's phospholipid bilayer (the layer that surrounds the entire cell), enters the target cell, and travels through the cytoplasm (the liquid-filled space inside the cell) toward the androgen receptor.
Once it reaches the receptor, testosterone binds to it and forms the proper conformation, called a receptor complex.
The complex (or 'receptor complex') is the receptor and hormone that are now bound together.
Once this happens, the complex travels to the nucleus of the cell and activates specific DNA sequences.
These specific DNA strands/sequences depend on the intended effect of testosterone on the cell and are called 'hormone response elements'.
For example, in muscle cells, testosterone activates gene transcription (the copying and reading of specific DNA code), which instructs the cell to begin synthesizing and building contractile proteins that ultimately increase strength and muscle size.
In layman's terms, testosterone acts as a cell's "vesicle" - it goes inside the cell, opens the container inside the cell that contains the instructions/blueprints for the cell to perform a specific task, and tells the cell to perform that specific task.
Using muscle cells as an example, it tells the muscle cells to start growing new muscle tissue.
Within different cell types, androgen receptor interactions initiate different cellular responses to the "reading" of these gene sequences.
Within muscle cells, testosterone not only tells the cell to start making new muscle cell components (actin and myosin, which are contractile proteins), but it also triggers the expression of gene transcripts with other functions, such as telling the muscle cell to increase its ability to store carbohydrates in the form of glycogen (a form of carbohydrate that can be used for muscle energy).
After this signaling process, testosterone can either dissociate (separate) from the receptor, destroying the receptor complex, and remain in the cell and re-engage in receptor binding, or it can leave the cell and circulate in the bloodstream.
However, it is very important for the reader to understand that the entire process of all these steps is a slow process and takes place over several hours.
Several studies have demonstrated that, on average, it takes about 4-6 hours for anabolic steroid hormones to dissociate from their intracellular receptors [3].
These same studies have also demonstrated that new androgen receptors are created after the hormone detaches from the receptor, which is very strong evidence that androgen receptor sites actually increase in the presence of androgens, and indicates that the previous theory of years past that androgen receptor "saturation" is the main reason for the decreased progression of anabolic steroid cycles is clearly not true.
An example of another body part that testosterone and related androgens affect (albeit with different end results) is the kidneys, where testosterone, through the same signaling via receptor interactions, signals kidney cells to start or increase production of a hormone called erythropoietin (EPO), which is a protein hormone that travels to the bone marrow and signals increased red blood cell production [4].
This is why anabolic steroid cycles lead to increased red blood cells and elevated hemoglobin levels.
"Hemoglobin levels" and "red blood cell count" are synonymous with each other because hemoglobin is a protein contained in the center of red blood cells, which is where oxygen attaches when red blood cells travel to the lungs to absorb oxygen. The red blood cells then travel to various tissues and cells in the body to deliver the attached oxygen, before returning to the lungs to allow more oxygen to attach to the hemoglobin protein.
Therefore, an increase in hemoglobin levels will always correlate with an increase in red blood cell levels.
In fact, all anabolic steroids exhibit this erythropoietic effect, and while some anabolic steroids stimulate red blood cell production more than others, they all exhibit this property, which is the result of the interaction of the anabolic steroid with the androgen receptors on the kidney cells.
However, there is evidence that dihydrotestosterone (DHT) and some derivatives may not exhibit this activity in kidney cells, because DHT is metabolized very rapidly to inactive hormones by enzymes of the 3-alpha hydroxysteroid dehydrogenase family, which are present in very high amounts in the kidneys as well as muscle tissue [5].
This 3-hydroxysteroid dehydrogenase enzyme is present in large amounts in muscle tissue (and certain other tissues in the body, such as the kidneys) and is responsible for metabolizing DHT into two inactive metabolites, 3-alpha-androstanediol and 3-beta-androstanediol.
These two hormones, which are metabolites of DHT, are not anabolic at all in muscle tissue.
Therefore, many chemists and biologists believe that DHT could actually be a very potent and powerful anabolic steroid if this enzyme, 3-hydroxysteroid dehydrogenase, were not present in muscle tissue.
The main advantage that most DHT derivatives have in this situation is that they contain a chemical modification that allows them to bypass this restriction so that the hormone does not interact with 3-hydroxysteroid dehydrogenase and is not metabolized by this enzyme.
As a result, DHT analogs can enter muscle tissue (or kidney tissue) and exert powerful anabolic effects on these cells.
Fat cells also respond to androgens such as testosterone, and while this does not cause dramatic fat loss, it does initiate lipolysis (breakdown of fat) in fat cells [6].
It has also been shown to increase the amount of androgen receptors on fat cells, the interaction of androgens with androgens, and the amount of beta-adrenergic receptors [7].
That testosterone and related anabolic steroids play an important role in regulating body fat levels can be seen from the fact that when testosterone and androgen levels decrease, for whatever reason, body fat levels generally increase [8].
This is very evident in women, who, as mentioned earlier, have much lower androgen levels than men, but estrogen also plays an important role in body fat storage, as estrogen not only increases the promotion and/or retention of body fat in certain areas of the body[9], but also in patients with hypogonadism and male menopause, who tend to experience increased body fat storage while testosterone levels decrease.
This is one of the reasons why athletes and bodybuilders who use anabolic steroids use a variety of methods to reduce or control estrogen levels during an anabolic steroid cycle.
Conversely, when androgen levels increase, the ratio of body fat to lean body mass decreases, resulting in a body composition that is characterized by an increase in lean body mass and a decrease in fat mass.
Other cells and tissues in the body where testosterone affects androgen receptors include the sebaceous glands in the skin, hair follicles in the skin, scalp, prostate, and certain other areas.
Areas such as the scalp, skin, and prostate are known as androgen-responsive tissues.
This means that these tissues are specifically responsive to androgens, and to a greater degree than other tissues.
The interaction of testosterone with cells in these tissues results in the secondary male sex characteristics experienced during puberty.
These include an increase in body and facial hair and an increase in sebum secretion from the skin associated with the sebaceous glands[10], which is why acne can be a problem during puberty and anabolic steroid cycles in individuals who are sensitive to the androgenic activity of these tissues.
In particular, these androgen-responsive tissues are designed to be more sensitive to dihydrotestosterone (DHT), a more potent androgen that causes testosterone to be reduced (or converted) in these tissues.
The scalp, skin, and prostate have high levels of 5-alpha reductase (5AR), an enzyme that converts testosterone to DHT.
DHT binds much more strongly to the androgen receptor than testosterone, making it an overall more potent androgen in these tissues.
When it comes to scalp and hair loss, a common concern for men, it's important to understand that in order for hair loss to occur due to increased androgen levels, you must have the genetic trait of male pattern baldness (MPB).
As mentioned earlier, testosterone and its potent metabolite DHT can bind to androgen receptors located on the scalp, which can cause male pattern baldness in individuals who have the genetic traits necessary for it to manifest, and this can occur in men as well as women.
Those who do not possess the specific gene required for activation will not experience this effect at all, at any dose.
This is why some people can use the strongest androgenic anabolic steroids, such as trenbolone, repeatedly for years and still have a full head of hair well into old age, while others experience hair loss even with very mild androgenic compounds.
Testosterone does not cause hair loss, nor do any anabolic steroids.
An individual's genetic inheritance causes hair loss, and testosterone only serves to accelerate the hair loss process that is already present.
Medical uses of testosterone and testosterone replacement therapy (TRT)
Testosterone and its related derivatives and analogs have many medical uses in treating an almost infinite amount of conditions, debilitations, and diseases.
In fact, there are so many applications that it would require a separate article focused solely on the medical use of testosterone and anabolic steroids, and it is not possible to cover them all here.
Testosterone and its analogs have been effectively utilized to treat hypogonadism and male menopause, female breast cancer, hereditary angioedema, anemia, muscle-wasting diseases such as AIDS/HIV, burn victims, muscle atrophy, male infertility, adolescent growth disorders, osteoporosis, female libido problems, Turner and Klinefelter syndromes, menopause, chronic dysfunctional uterine bleeding (menorrhagia), endometriosis, and numerous other conditions.
It is very clear that anabolic steroids are miracle drugs in the medical world, and they have the ability to be utilized in the treatment of almost any medical indication because of the systemic effects they have on the body, which makes them applicable to an infinite number of uses and treatment options.
Anabolic steroids and testosterone have been used for both medical and off-label purposes over the past 70 years, helping to save hundreds of thousands of lives.
Medically, the main purpose of testosterone itself is specifically for testosterone replacement therapy (TRT), also known today as hormone replacement therapy (HRT) and androgen replacement therapy (ART).
TRT is a treatment typically administered to individuals who have lower than normal levels of testosterone production for a variety of reasons.
The common and appropriate medical term for this condition is known as hypogonadism, which is a condition in which an individual's testicles produce testosterone inadequately, which can be caused by genetics, physical injury, disease, or other reasons.
The specific medical term for individuals suffering from age-related decline in adequate testosterone production is known as andropause.
The treatment of hypogonadism and male menopause is probably the most common purpose for the use of testosterone in medicine worldwide.
It's a simple treatment that can be easily defined as the supplementation of testosterone to restore a man's testosterone levels to a normal or high range.
The majority of patients receiving testosterone therapy for this purpose are adult males with an average age of over 30 years old, which is very strong evidence to suggest that most of them are male menopausal patients (patients with decreased testosterone levels due to aging).
Early surface signs and symptoms of low testosterone are easy to recognize: a significant decrease in sexual function and libido (including erectile dysfunction), a significant loss of overall daily energy, decreased muscle strength and overall physical performance, depression (to varying degrees), increased bone weakness, decreased motivation, memory loss, and muscle loss [11].
These symptoms occur in both those experiencing andropause as well as those with generalized hypogonadism.
In fact, male menopause is, for all intents and purposes, a subcategory of hypogonadism.
It is simply a variation of hypogonadism and can be categorized as age-related hypogonadism.
While the symptoms listed above are generally very strong indications that your testosterone levels are lower than normal, there is usually a general reference range for what your testosterone levels should be for easy reference and judgment by your doctor.
This range will vary depending on which healthcare professional is monitoring your blood levels and which healthcare organization in which country sets the standards, but it is important to understand that even when a reference range is provided, it is not fixed and doctors should always be able to treat patients based on their own unique assessment and judgment of the situation.
Therefore, the average 'normal' endogenous testosterone range is considered to be between 350 ng/dl and 850 ng/dl.
As mentioned earlier, the range of 350 - 850 ng/dl can vary (for example, some medical literature suggests a range of 241 - 827 ng/dl in some countries/regions/organizations).
In general, the range of 350 - 850 ng/dl is considered to be the normal range.
While this 350-850 ng/dl range is considered the "normal" range, there are low, medium, and high values within this normal range. For example, a person with a plasma testosterone level of 750 ng/dl is said to be in the upper end of the normal range.
A testosterone level of 430 ng/dl is said to be in the lower end of the normal range.
A level below 350 ng/dl is considered a serious problem (as mentioned earlier, some medical professionals lower this minimum level to 241 ng/dl or even lower).
As you can see, the threshold for what is considered a "normal" level of testosterone is actually quite large.
This is one of the reasons why hypogonadism is actually a very under-diagnosed condition, and why many doctors are still reluctant to prescribe testosterone for a variety of reasons (some of which could be considered unreasonable).
It is important for individuals who suspect they have hypogonadism to find a doctor who understands hypogonadism, is reasonable, and is willing to work with the patient.
Most doctors are not familiar with and untrained in hypogonadism and will (foolishly) choose not to address the issue at all.
After identifying your symptoms, it's important to get a blood test to be 100% sure that your testosterone levels could actually be low.
Until you have a proper blood test done to check your testosterone levels, you can't be absolutely sure if you have hypogonadism or not.
Testosterone replacement therapy has a high success rate in alleviating all of the aforementioned symptoms because it raises testosterone levels to above the normal range.
The range of relief from various symptoms with TRT varies. For example, sexual dysfunction and libido are corrected when levels return to 350 ng/dl or higher.
After six months, 250 mg of testosterone enanthate every 21 days has been shown to increase bone mineral density by 5% [12].
This is a perfect example of how administering testosterone to restore normal physiological levels can help restore and improve the functioning of a variety of systems, as discussed earlier in this article where we described the effects of testosterone on a cellular level.
This includes an increase in red blood cell count, which leads to increased endurance, improved energy, well-being, and restored muscle mass.
There are various studies that have determined where testosterone levels should be, on average, for men of different ages [13].
These studies determined the median plasma testosterone levels by age as follows
It's important to remember that these numbers are not set in stone and can actually vary greatly between individuals.
It is very important to work with a doctor who understands and interprets these functions and results properly.
For example, total testosterone is very different from free testosterone (free testosterone is the amount of testosterone in your plasma that is not bound to SHBG and is free to act in your body).
For those unfamiliar with it, SHBG is a protein that binds to sex hormones (testosterone, estrogen, etc.) and renders them inactive, acting as a "handcuff" to the hormones, preventing them from doing anything as they float around in the bloodstream.
Low testosterone levels are associated with the symptoms described above, as well as a number of other health complications and risks.
One of the risks for men with low testosterone levels is an increased risk of cardiovascular disease (CVD).
Studies have shown that testosterone used for TRT purposes produces positive changes in blood lipid (cholesterol) profiles, with decreases in 'bad' LDL cholesterol and total cholesterol, and no change in 'good' HDL cholesterol [14] [15].
In general, as testosterone levels begin to reach the supraphysiologic dose range, cholesterol changes begin to shift slightly negative.
In addition to these positive changes, restoring testosterone levels to the normal-to-high range has been proven to reduce adipose (fatty) tissue around the midsection, lower obesity rates, and improve the body's sensitivity to insulin and blood sugar control [16].
The opposite of these improvements often leads to metabolic complications such as diabetes and obesity, which increase the risk of cardiovascular disease.
Testosterone supplementation may also reduce inflammation in the body by increasing levels of the anti-inflammatory cytokine IL-10 and decreasing the pro-inflammatory cytokines TNF-alpha, IL-1 beta, and IL-6 [17].
Not only does this help reduce inflammation systemically, but it also helps protect the artery walls from the same effects of inflammation.
Of course, there are some risks and potential side effects associated with TRT.
The list of side effects associated with testosterone supplementation is the same as the list of side effects associated with testosterone use, but older men in particular seem to experience more potential side effects than younger TRT patients.
Because testosterone is an androgenic hormone, it has been found to cause benign prostatic hyperplasia (BPH) in some individuals and can increase prostate-specific antigen (PSA) levels [18] [19].
While an enlarged prostate is not usually a health or life-threatening issue, it can be uncomfortable for many people, and while testosterone and related androgens (e.g. DHT) are not the cause of prostate cancer, they can play a role in the development of prostate cancer if the environment is perfectly positioned for that risk, along with other causative hormones.
Normal, healthy men who did not have prostate problems prior to TRT typically do not develop these problems, so it is generally recommended that men with prostate cancer, or who have had prostate cancer in the past, and/or men with generally high PSA levels avoid testosterone use [20].
Prostate problems caused by testosterone or anabolic steroid use appear to be a matter of age and, of course, heredity.
This was demonstrated in a 2001 study that looked at supraphysiologic (bodybuilding, not TRT doses) doses of testosterone (600 mg weekly for 20 weeks) in 61 healthy men aged 18-35 years and found no significant effect on prostate-specific antigen (PSA) [21].
More information on how TRT is administered and treated, as well as monitoring blood test results while on TRT, will be covered in more detail in this article.
Athletic performance enhancement and other non-medical uses of testosterone
To date, the use of testosterone for the purpose of athletic performance and physique enhancement is (ironically and hypocritically) an unapproved use throughout the medical community.
This is highly ironic considering that the first use of testosterone was for physical and athletic performance enhancement, and the first officially created testosterone analog, Dianabol (methandrostenolone), was not designed for any purpose other than to be used as an athletic performance enhancing drug.
When testosterone was first synthesized in the 1930s, the eyes, ears, and undivided attention of the entire medical community were focused on its development and discovery.
Almost everyone in medicine believed, and rightly so, that the uses and applications of this substance would be unimaginably wide and far-reaching.
These people were right: testosterone and anabolic steroids were first and foremost used for the purpose of enhancing athletic performance and physique, and the only reason they are not accepted by the medical community today is because of political agendas, political reasons, and political manipulation of the medical community in relation to the issue of "cheating" in professional sports.
However, the use of anabolic steroids for the purpose of enhancing athletic performance and physique is literally the most prevalent use, and far exceeds any other use in medicine.
Despite being illegal worldwide, athletic performance enhancement remains the primary purpose of anabolic steroid use.
In fact, studies have shown that the average anabolic steroid user is actually a middle-class, heterosexual male with an average age of about 25-35 years old, who is neither a competitive bodybuilder at any level nor a professional or amateur athlete at any level, and that these anabolic steroid users are simply using anabolic steroids for cosmetic enhancement purposes [22].
These findings from many studies have dispelled the rumors and misconceptions that anabolic steroid users for performance enhancement purposes consist of only professional athletes or teenage athletes in high school.
Testosterone is used by people who want to maximize their athletic performance and physique by increasing the levels of a hormone (testosterone) already present in the body.
It's pretty simple: Testosterone at supraphysiological levels (bodybuilding doses) can lead to greater and faster gains in strength and size, provided that nutrition and training are properly adjusted for this effect.
It is also used by bodybuilders and individuals who are losing fat to maintain muscle tissue that is being broken down during periods of dieting (calorie deficit).
Because anabolic steroids help to increase strength, increase muscle size, and decrease body fat levels, they are favored not only by athletes but also by individuals looking to improve their appearance.
Testosterone provides individuals who have reached their genetic limit for gaining maximum muscle mass the ability to surpass their genetic limitations.
Contrary to the common misconception that testosterone and related anabolic steroids are used as a shortcut, testosterone is often used for this purpose.
There are other, less popular non-medical uses for testosterone, such as its libido-enhancing effects.
In fact, for a variety of reasons, some people want to use testosterone for just one or two of its effects on the body that have nothing to do with its ability to increase muscle mass.
However, among the off-label (non-medical) uses of testosterone, the use of testosterone to enhance athletic performance and physique is by far the most popular.
It is even the most widely used use compared to all uses of testosterone (medical and non-medical).
Medical references:
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[19] Prostate volume and growth in testosterone-substituted hypogonadal men are dependent on a CAG repeat polymorphism in the androgen receptor gene: A longitudinal pharmacogenetic study. Zitzmann M, Depenbusch M, Gromoll J, Nieschlag E. J Clin Endocrinol Metab. 2003 May;88(5):204954.
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