Testosterone Life Cycle

Testosterone Life Cycle

Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.


For centuries, testosterone has remained a fascinating topic for researchers and individuals alike. This vital hormone plays a significant role in everything from reproductive health to muscle mass, energy levels, and even mood.

But understanding the life cycle of testosterone is key to unlocking its secrets and harnessing its potential. 

By gaining a better understanding of this hormone's life cycle, you can make informed decisions about maintaining optimal testosterone levels and improving your overall well-being. 

Testosterone, often dubbed as the "male hormone," plays a crucial role in various physiological functions, from puberty to the golden years.

In this eye-opening article, we will delve deep into the secrets of the testosterone life cycle, uncovering the fascinating transformations that occur during different stages of life.

During puberty, testosterone surges, triggering a cascade of physical changes like deepening of the voice, muscle growth, and the emergence of facial and body hair.

As men enter adulthood, testosterone levels stabilize, providing fertile ground for reproduction and maintaining overall health.

However, as the years pass, testosterone gradually declines, leading to noticeable changes in energy levels, libido, and cognitive function.

Join us as we unlock the mysteries of the testosterone life cycle, enabling you to gain insights and make informed decisions about your health and well-being. 

  • Testosterone
  • The life cycle of testosterone
  • Plasma Cycle
  • Deficiency
  • Pregnancy
  • Testosterone levels in infancy to puberty
  • Testosterone in adulthood
  • Managing testosterone
  • Testosterone replacement therapy
  • Conclusion


Testosterone is an essential male hormone that impacts everything from sperm production and bone and muscle mass development to fat storage and mood. Normal levels can fluctuate greatly.

Testosterone, the main male steroid hormone, is produced in two major locations: Leydig cells of the testes and, to a lesser degree, by ovaries. Furthermore, it's produced in lesser amounts in women's adrenal glands as well.

Free Testosterone

Lipophilic hormones such as testosterone and steroid hormones (soluble only in lipids) are transported through water-based plasma by specific and nonspecific proteins.

SHBG, a specific protein, binds sex steroids such as testosterone, estradiol and dihydrotestosterone.

Albumin and lipoprotein are non-specific binding protein. Free hormones are the part of total hormone concentrations that are not bound to their respective carrier proteins.

Free testosterone is the testosterone that is not bound by SHBG. Only the free testosterone is able to bind with an androgenic receptor, meaning that they have biological action.

The binding of testosterone to plasma proteins is between 98.0 and 98.5% with 1.5 to 2.0% free or unbound

It is binded 65% to the sex hormone binding globulin, and 33% to albumin. 


Men can rely on testosterone to fuel sexual drive, enhance muscle strength and regulate mood.

Additionally, this hormone has been documented to cause seasonal increases in aggression - something well-documented by studies on elephant seals, zebras and even Australian wild camels that are currently in sexual heat.

Testosterone belongs to a class of hormones called androgens and has often been associated with "male" sexual drive; however, this hormone can benefit both sexes equally.

The body produces testosterone in Leydig cells of the testicles from cholesterol. Once converted, androstenedione binds with androgen receptors on target tissue cells to produce its effects.

An appropriate balance of androgens is crucial for normal reproductive function.

A hypothalamus-pituitary-gonadal axis regulates the production and release of testosterone as well as the release of gonadotropin-releasing hormone and luteinizing hormone.

These hormones stimulate testicles to produce testosterone and initiate spermatogenesis; testosterone also contributes to bone and muscle strength as well as protection from various diseases including cardiovascular disease and dementia.

Testosterone is a Steroid

The steroid testosterone is an androstane steroid that contains a hydroxyl and a ketone group respectively at position three and seventeen.

It is produced in a number of steps by cholesterol, and then converted to inactive metabolites in the liver. 

Its action is mediated by binding and activation of androgen receptors. The testicles and ovaries are the primary sources of testosterone in humans and other vertebrates.

In adult males the levels of testosterone tend to be seven-to-eight times higher than in adult females.

The daily testosterone production in men is approximately 20 times higher than in women. This is due to the fact that the male metabolism of the hormone is more prominent. 

Testosterone Levels

The total levels of testosterone have been reported to range from 264 to 916 nanograms per deciliter (nanograms/deciliter) among non-obese Europeans and Americans aged 19 to 39, whereas the mean testosterone level in adult males has been reported to be 630 ng/dL. 

While this range is commonly used to measure hypogonadism, certain physicians dispute its use. 

Multiple professional medical groups recommend that 350 ng/dL be generally considered as the minimum level of normal, which is in line with previous findings. The levels of testosterone in males decrease with age.

The mean testosterone levels in women have been reported at 32.6 ng/dL. The mean testosterone levels in women with hyperandrogenism were reported at 62.1 ng/dL.

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The Testosterone Life Cycle

Testosterone is the primary male sexual hormone.

Testosterone, the most important bioactive androgen in the testis and circulation of males, is the dominant reproductive hormone.

Reduced levels of testosterone in the testes may result in a reduction in sperm count.

It promotes the development of both internal and external reproductive organs, muscle growth and red blood cell production; increases aggression, the desire to impress others as well as an enhanced libido and sense of well-being.

Testosterone is produced in Leydig cells of the testicles. Beginning as cholesterol, is converted by your body into androstenedione or "testosterone," then acts as a precursor to other androgens such as dihydrotestosterone (DHT), which may cause acne on skin, and thin out hair follicles.

Leydig cells are the primary testosterone-producing cells in males. Leydig cells that mature into adulthood are visible at puberty, and the majority of them persist without changing.

It has been demonstrated that the rodents' differentiation of stem Leydig and progenitor Leydig from the stem Leydig occurs between days 10 to 14 postnatally. This marks the beginning of Leydig lineage.

By approximately day 28 postpartum, the progenitor Leydig cells begin to transition into immature Leydig cells, and these cells then divide once or twice and differentiate into testosterone-secreting adult Leydig cells.

Adult testis also contain stem Leydig cells. As men and rodents age, testosterone levels in the serum decrease.

This is not due to the loss of Leydig cell but rather a reduced ability to produce steroidogenesis.

Though the mechanism by which reduced testosterone production by Leydig cells remains uncertain, there is strong evidence to support an imbalance in the aging Leydig cells' prooxidant/antioxidant environment as playing a central role. 

Plasma Testosterone Cycle

This study aimed to determine the stability of testosterone levels within plasma samples of males in normal health over time, and to look for patterns in the changing levels.

The total testosterone concentration was determined by radioligand-saturation analysis using late-pregnancy blood samples from 20 healthy young males collected every 2nd day over a period of 2 months.

The coefficients of variations ranged between 14 and 42% (median 22%); for the majority of individuals, the fluctuations of testosterone plasma levels were significant.

Four different and relatively independent methods were used to test for the presence of periodic functions.

Twelve out of 20 subjects showed a high degree of agreement between at least three analytic methods. The 12 subjects showed cycles in plasma testosterone levels ranging from 8-30 days with a cluster around 20-22 day periods.

Most of these cycles were at least 5% significant. The average amplitude of these cycles was 17%. 

Testosterone Deficiency

Testosterone may be perceived as being associated with masculine behavior and bad boys, but its benefits go much deeper.

Not only does it fuel sex drive and muscle growth, it can also regulate mood and build bone strength while playing an integral part in prostate health.

Testosterone levels decline naturally as we age, known as andropause or male menopause.

Low testosterone can also be caused by an underlying health issue such as pituitary gland or hypothalamus dysfunction, damaged testicles (such as from mumps), or cancer treatments which damage these parts of the body that produce hormones.

Testosterone production can be increased using supplements, shots, gels and patches.

Studies have demonstrated the benefits of testosterone replacement therapy can improve libido, increase muscle mass and protect bones; reduce symptoms of depression as well as enhance sexual performance.

A good plan should be carefully created with the assistance of a physician as the effects can vary among men; injections usually begin working within four weeks with full effects becoming evident six-12 months later.


The level of androgen is related to sex development during the second trimester. 

Specifically, testosterone along with antimullerian hormone (AMH), promotes growth of the Wolffian canal and degeneration in the Mullerian tube respectively. 

This period can affect the gender of a fetus. It is able to predict more accurately the behaviour of a sexed fetus than the adult's level.

The prenatal androgens influence gendered interests and behaviors and may have a moderate effect on spatial abilities.

Testosterone plays a role in the primary sexual development. This includes spermatogenesis and the enlargement and enlargements of the penis, testes and libido.

Around 7 months after conception, the testes start to secrete a reasonable amount of testosterone.

At approximately week seven in utero, male infants begin developing testicles under the control of a gene on the Y chromosome.

Testicle production then triggers internal and external reproductive organ development during fetal development as well as secondary male characteristics such as facial hair growth, body hair, deep voice tone and facial hair thickness.

Testosterone also stimulates penis, testes and prostate gland growth as well as stimulating skeletal muscle growth while helping regulate your metabolic rate by burning off more fat as fuel for fuel than normal.

Your body produces dehydroepiandrosterone (DHEA), which your adrenal glands convert into testosterone and estrogen for sexual chemistry.

While natural levels of these sex hormones vary based on gender and body weight, AMABs typically have higher testosterone concentrations compared with AMFABs.

At each stage in its metabolism, testosterone undergoes numerous transformations including oxidation, conjugation via sulfation and glucuronidation.

Once this process is completed, its chemical structure allows it to interact with receptors located throughout the brain and other tissues.

At the end of testosterone's lifecycle is its conversion into estradiol - the main female hormone. This usually takes place within fat cells. 

Testosterone Levels in Infancy to Puberty

The effects of early infancy androgens are poorly understood. The testosterone levels of male infants rise in the first few weeks of their lives.

After a few weeks, the levels are still in a pubertal level. However, they reach the barely detectable childhood levels by 4-7 month of age. 

It is not known what this increase in human height means. Theorists have suggested that the brain is masculinizing, since there are no other significant changes in the body. 

The aromatization process of testosterone to estradiol masculinizes the male mind. Estradiol crosses the blood brain barrier and enters into the male brain. Female fetuses, however, have a-fetoprotein which binds estrogen, so the female brain is not affected.

As children age from 8-13, both girls and boys experience physical and emotional changes known as puberty.

While these changes come about gradually and vary from child to child, hormones released from the brain start this process in both genders.

In males, these signals stimulate testicles to release testosterone which then travels through blood in two forms:

  1. Tightly bound to protein sex hormone-binding globulin 
  2. Free and unbound

Only the latter type is biologically active so often when levels rise it will be bioavailable testosterone rather than tightly bound forms sex hormone-binding globulin form as opposed to bioavailable forms (shortened to STB).

Men in their teenage years experience their highest levels of free and unbound testosterone production - approximately six milligrams daily.

Teenagers undergoing this hormonal surge tend to become talkative and energetic.

Furthermore, high levels of testosterone increase assertiveness and competitiveness as well as sexual curiosity - possibly accounting for the spike in teen pregnancy rates as well as sexually transmitted diseases like gonorrhoea.

This marks a change from the soprano voices of teenage boys to deeper, deeper tones as muscle replaces baby fat and interest in sex and libido reach their highest points.

The following effects are common in males and women who have experienced prolonged levels of elevated free testosterone in their blood. These effects include:

  • The testicles grow spermatogenic tissues, male fertility is increased, the penis or clitoris are enlarged, and there's an increase in libido, frequency of erections, or clitoral swelling.
  • An increase of human growth hormone is responsible for the growth of jaw, brows, noses and chins as well as remodeling of facial bones contours. 
  • The maturation of the bones and growth of the bone is complete. It occurs in men more slowly than women, and indirectly through estradiol-metabolites.
  • The Adam's apple grows, the voice deepens, and shoulders and ribcage expand.
  • Sebaceous glands enlargement. Subcutaneous fat on the face may decrease.
  • Increased chest hair, periareolar, perianal, leg, and armpit hair.

Testosterone in Adulthood

Dehydroepiandrosterone (DHEA), produced in the adrenal gland, is converted to testosterone and estrogen by enzymes in both testicles and prostate glands.

Once released into bloodstream, this testosterone enters various tissues for various effects.

The brain sends signals to the pituitary gland, which regulates how much testosterone is produced.

A feedback loop then helps manage its levels in the bloodstream.

Too little testosterone production, known as hypogonadism, and too much production known as hyperandrogenism both lead to conditions known as hypogonadism or hyperandrogenism respectively.

Testosterone is essential for the normal development of sperm. It activates Sertoli cell genes that promote spermatogonia differentiation.

It regulates acute HPA (hypothalamic-pituitary-adrenal axis) response under dominance challenge.  Androgens including testosterone enhance muscle growth.

The testosterone hormone also regulates platelet aggregation by regulating the number of thromboxane A2 receptors on megakaryocytes. 

The effects of adult testosterone are more obvious in males but they are important for both sexes. These effects could diminish as testosterone levels decrease during adulthood. 

This sexual differentiation also affects the brain. The enzyme aromatase is responsible for the masculinization of male mice's brains.

In humans, the masculinization of fetal brain is associated with functional androgens receptors, as observed by gender preference among patients with congenital disorders in androgen production or receptor function. 

The Role of Testosterone in Aging

Testosterone levels rise during prenatal development and puberty before becoming stable throughout adulthood.

While female hormone levels fluctuate dramatically during ovulation and menopause, male testosterone levels begin to slowly diminish from age 30 onward.

From that time forward, testosterone levels in men may drop by around one percentage point each year until death when their levels will likely become low enough to be considered low enough for treatment purposes.

Low testosterone has been linked with slower rates of mental decline later in life for men than for women.

This effect may not be as dramatic; diet, exercise and stress management may help reverse it.

Low testosterone levels can cause mood disturbances, loss of muscle mass and weakness as well as poor erections and sexual dysfunction.

Low testosterone has even been linked with decreased bone density and an increased cardiovascular risk; replacement therapy can improve these symptoms while still offering good safety profiles in hypogonadal men.

Ensuring enough testosterone throughout later life may protect men against osteoporosis and the development of Alzheimer's disease.

Managing Testosterone Levels

The amount of testosterone synthesized is regulated by the hypothalamic-pituitary-testicular axis. 

When testosterone levels are low, gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, which in turn stimulates the pituitary gland to release FSH and LH.

These two hormones stimulate testis to produce testosterone.

Finaly, an increase in testosterone levels through a negative feed-back loop acts on the pituitary and hypothalamus to inhibit GnRH release and FSH/LH respectively.

The following factors can affect testosterone levels: 

  • Resistance training increases testosterone acutely, however, this increase can be prevented in older men by eating protein. Men who train for endurance may have lower testosterone levels. 
  • Vitamin A deficiency can lead to low levels of plasma testosterone. Secosteroid Vitamin D at 400-1000 IU/d (10-25 ug) increases testosterone levels. Zinc deficiency reduces testosterone levels, but excessive supplementation does not affect serum testosterone. Low-fat diets can reduce testosterone in men, but there is little evidence to support this. 
  • Weight loss can increase testosterone levels. The enzyme aromatase is synthesized by fat cells, and this enzyme converts testosterone (the male sex-hormone) into estradiol (the female sex-hormone). There is no correlation between testosterone levels and body mass index. 
  • Sleep (REM sleep) increases nighttime testosterone levels.
  • In some cases, dominance challenges can stimulate testosterone release in males.
  • Spearmint tea, for example, is a natural or synthetic antiandrogen that reduces testosterone levels. Licorice is a natural antiandrogen that can reduce testosterone production. This effect is more pronounced in women. 

Testosterone Replacement Therapy

Testosterone replacement therapy has become more and more prevalent as men age, according to The New England Journal of Medicine.

While testosterone gets an unfavorable reputation due to bodybuilders and cheating athletes abusing its use, doctors point out that its benefits far outweigh its risks when used responsibly.

As the first step toward increasing testosterone levels, it's essential to rule out any health conditions that might be responsible, such as pituitary gland issues or prostate cancer.

A blood test measuring luteinizing hormone and prolactin levels may reveal potential issues.

If your doctor determines that you have low levels of testosterone, he or she may suggest testosterone treatment via skin gels or intramuscular injections.

They will discuss each option's potential pros and cons with you before suggesting an approach that's best suited to you.

Treatment with TRT can bring their levels back into line and improve overall cardiovascular health by strengthening bones, increasing muscle mass, decreasing fat accumulation and strengthening overall cardiovascular well-being.

TRT may help increase libido and sexual function, reduce fatigue, depression, and irritability, as well as decrease symptoms quickly after initiating treatment. Results should usually become apparent within several weeks.

TRT may be effective in treating medical conditions like late-onset hypogonadism, but should never be the sole form of treatment a patient seeks. 


Testosterone often gets the blame for fueling aggressive behavior and fueling masculinity in men. But testosterone plays an essential role in driving sexual desire, muscle strength and bone health - as well as helping regulate mood.

Testosterone production increases during foetal development, peaking during puberty and gradually declining thereafter in both men and females.

Men have an active "feedback loop" designed to maintain optimal levels by sending signals from their brain to the pituitary gland - specifically the hypothalamus releasing gonadotropin-releasing hormone (GnRH), which then stimulates production of luteinizing hormone and follicle stimulating hormone.

A low level of this sex hormone could even contribute to low sperm counts or infertility issues in some cases. Women's ovaries and adrenal glands are sources of testosterone production as well.

When levels fall below their recommended ranges, US physicians may prescribe injections or gels as treatments.

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