Quick Answer
Testosterone is primarily produced and stored in the testes in males. The testes contain Leydig cells that synthesize testosterone from cholesterol. After production, testosterone leaves the testes and enters the bloodstream, where a small amount circulates freely while the majority is bound to carrier proteins like albumin and sex hormone-binding globulin. The adrenal glands and peripheral tissues like fat and muscle also produce small amounts of testosterone.
Where is testosterone produced in males?
Testosterone is predominantly produced in the testes in males. The testes contain clusters of specialized cells called Leydig cells that synthesize testosterone from cholesterol through a series of chemical reactions.
Leydig cells possess the enzymes required to convert cholesterol into testosterone. These reactions begin with the transport of cholesterol into the mitochondria of Leydig cells. Through a process called steroidogenesis, cholesterol is converted to pregnenolone. Pregnenolone is then converted to testosterone through two pathways:
- Pregnenolone -> Progesterone -> Androstenedione -> Testosterone
- Pregnenolone -> 17α-hydroxypregnenolone -> Dehydroepiandrosterone (DHEA) -> Androstenediol -> Testosterone
The rate-limiting and regulated step in testosterone synthesis is the conversion of cholesterol to pregnenolone by an enzyme called cytochrome P450 side chain cleavage enzyme (P450scc).
In addition to the testes, smaller amounts of testosterone are also produced in the adrenal glands and peripheral tissues like fat and muscle in men. The total contribution from the adrenal glands and peripheral tissues is less than 5% of total testosterone production in males.
Testosterone production in the testes
The testes are responsible for around 95% of testosterone production in men. Testosterone synthesis takes place in specialized cells called Leydig cells that reside in the loose connective tissue between the seminiferous tubules of the testes.
During puberty, increased gonadotropin release from the pituitary gland stimulates testosterone production by Leydig cells. The number of Leydig cells increases, and the activity of steroidogenic enzymes involved in testosterone synthesis intensifies. By early adulthood, Leydig cells number about 700 million and produce 3-10 mg of testosterone daily.
Testosterone production varies in a circadian rhythm, with peak production occurring in the early morning. Testosterone levels can also rise in response to sexual arousal or physical activity. Temperature and local growth factors regulate testosterone synthesis over the long term.
Testosterone production in the adrenal glands
The adrenal glands sit above each kidney and consist of an outer cortex and inner medulla. The cortex produces small amounts of testosterone and other weak androgens like dehydroepiandrosterone (DHEA) from cholesterol.
The zona reticularis layer of the adrenal cortex contains cells that express many of the same steroidogenic enzymes found in Leydig cells. These enzymes allow adrenal cells to convert cholesterol into the testosterone precursor androstenedione.
However, adrenal cells lack 17β-hydroxysteroid dehydrogenase, the enzyme that converts androstenedione into testosterone. As a result, directly secreted testosterone makes up less than 5% of adrenal androgen production in men. Still, the adrenal contribution can help maintain testosterone levels, especially if testicular production is impaired.
Testosterone production in peripheral tissues
Many peripheral tissues like fat, skin, bone, and muscle contain low levels of the enzymes required to produce sex steroids like testosterone from cholesterol or steroid precursors. The testosterone synthesized in peripheral tissues is referred to as “intracrine” testosterone.
This localized testosterone acts within the same cell or neighboring cells where it was produced. It does not enter the bloodstream but plays important paracrine and autocrine roles regulating metabolism, growth, and function within those tissues. Still, peripheral testosterone only accounts for about 5% of total testosterone production.
Where is testosterone stored in males?
After synthesis, testosterone quickly moves out of Leydig cells into the lymphatic vessels and blood supply of the testes. From there it enters the systemic blood circulation.
Instead of being stored, most testosterone released from the testes is either transported bound to proteins in the blood or taken up by tissues throughout the body.
Circulating testosterone
Only 1-3% of testosterone in the bloodstream is unbound or “free” testosterone. The rest is bound to plasma proteins like sex hormone-binding globulin (SHBG) and albumin.
SHBG binds about 50-60% of circulating testosterone with high affinity. Albumin binds about 40% with lower affinity. The remaining 1-3% of testosterone not bound to SHBG or albumin is considered free testosterone.
Free and albumin-bound testosterone are referred to as “bioavailable” testosterone, as these fractions readily dissociate to allow uptake into target tissues. SHBG-bound testosterone acts as a reserve that maintains total testosterone levels.
Testosterone bound in tissues
Testosterone that dissociates from SHBG and albumin in the bloodstream can enter target tissues like muscle, fat, the skin, and the prostate gland. Within these tissues, testosterone can:
- Bind to and activate the androgen receptor, triggering genomic effects
- Be converted to metabolites like dihydrotestosterone (DHT) by intracellular enzymes
- Be degraded and inactivated
Some testosterone is stored long-term in tissues. This has been demonstrated in adipose tissue, where testosterone can be incorporated into the lipid fraction of fat cells. However, static storage makes up a small fraction of the overall testosterone pool compared to the more dynamic circulation between blood and tissues.
How is testosterone transported in the blood?
Testosterone has low solubility in aqueous solutions like blood. Instead, it is transported bound to carrier proteins. SHBG and albumin serve as the main binding proteins:
Sex hormone-binding globulin (SHBG)
SHBG is a glycoprotein produced mainly by the liver. It binds testosterone and other androgens with high affinity, slowing their distribution out of blood and extending their circulating half-life.
About 60% of testosterone is bound to SHBG at any given time. However, SHBG binding reduces the fraction of testosterone that is considered biologically active or “free”.
Conditions that increase SHBG like aging, liver disease, hyperthyroidism, and use of oral estrogens can lower free testosterone. Androgens and insulin inhibit SHBG production.
Albumin
The protein albumin makes up over half the protein content of blood plasma. Testosterone binds to albumin with lower affinity than SHBG, so a greater fraction remains unbound or “free”.
Albumin binding increases the solubility of testosterone in circulation and serves as a circulating reservoir. Approximately 40% of testosterone in blood is reversibly bound to albumin.
Levels of SHBG and albumin both impact the amount of free, bioavailable testosterone. Aging decreases SHBG and increases albumin, while obesity decreases albumin. These changes can maintain or even increase free testosterone in older men despite declining testosterone production.
Free testosterone
Free or unbound testosterone makes up 1-3% of total testosterone in circulation. This fraction has the highest biological activity as it readily diffuses into tissues and activates androgen receptors.
Equilibrium binding dynamics allow free testosterone to be replenished as it is taken up into tissues. Maintaining adequate free testosterone is essential for supporting normal masculine physiology and body composition.
How is testosterone converted and inactivated?
Circulating testosterone that reaches target tissues can be:
- Converted into more potent androgens like dihydrotestosterone
- Conjugated to form inactive metabolites
- Degraded into inactive products
These pathways locally regulate the amount of active testosterone within each tissue.
Conversion to dihydrotestosterone
The enzyme 5α-reductase converts testosterone into dihydrotestosterone (DHT) in tissues like the prostate gland, hair follicles, and fat. DHT has greater androgenic potency than testosterone.
This conversion amplifies the androgenic signal in key target tissues. It also serves to deactivate testosterone, as DHT is further metabolized before entering the bloodstream.
Conjugation
Testosterone can be conjugated or attached to molecules like glucuronic acid by UDP-glucuronosyltransferase enzymes (UGTs) expressed in the liver, kidney, intestine and other tissues. This produces inactive metabolites like testosterone glucuronide.
Conjugation makes testosterone more water soluble for excretion. Conjugated testosterone metabolites are eventually removed from the body in urine and bile.
Degradation
The enzymes 17β-hydroxysteroid dehydrogenase (17β-HSD) and 3α-hydroxysteroid dehydrogenase (3α-HSD) deactivate testosterone in tissues by converting it to inactive metabolites like androstenedione and androsterone. These degrade testosterone locally where it is not needed.
In the liver, testosterone can also be broken down entirely in a process called denaturation. This liberates the steroid A-ring as formic acid, leaving behind a degraded 17-keto steroid metabolite.
Factors influencing testosterone transport and metabolism
Many factors can alter testosterone production, transport, conversion, and breakdown. These include:
Aging
Testosterone production declines with aging, beginning after age 30. However, SHBG increases and albumin decreases, helping maintain free testosterone. Clearance may be reduced.
Obesity
Obesity is linked to low testosterone. Increasing adipose tissue aromatase can increase estrogen levels and reduce LH drive to the testes. Obesity also lowers SHBG.
Physical activity
Exercise transiently increases testosterone production but enhances androgen receptor expression and free testosterone over the long term. It increases musculoskeletal uptake and metabolism.
Medications
Glucocorticoids, opioids, and antipsychotics can reduce testosterone production. Estrogens increase SHBG. Steroid abuse bypasses normal feedback regulation.
Disease states
Primary and secondary hypogonadism directly impair testosterone synthesis. Liver and kidney disease disrupt carrier proteins and clearance. Some tumors overexpress inactivating enzymes.
Conclusion
In men, testosterone is primarily synthesized in the testicular Leydig cells and then quickly released into circulation. A small fraction circulates as free testosterone, while the majority is bound to SHBG and albumin proteins.
Testosterone enters target tissues where it can be converted to more potent androgens like DHT or inactivated. The rate of transport, delivery to tissues, metabolism, and clearance all significantly impact the bioavailability and action of testosterone throughout the body. Multiple physiological and pathological factors can disrupt this delicate balance.
Key Points
- The testes produce around 95% of testosterone in men, with the adrenals and peripheral tissues like fat providing the remainder.
- After synthesis in Leydig cells, testosterone is released into the lymphatic drainage and blood supply of the testes.
- Circulating testosterone is 60% bound to SHBG, 38% bound to albumin, and 1-3% is free.
- Free and albumin-bound testosterone are biologically active fractions that can enter tissues.
- Testosterone is converted to more potent DHT or inactivated by metabolism in target tissues.
- SHBG, albumin, metabolic enzymes, and other factors regulate testosterone transport, delivery, and effects.
