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Testosterone
Testosterone is the principal androgen produced by the testes, primarily by Leydig cells under stimulation from luteinizing hormone. It is essential for male reproductive development, spermatogenesis, secondary sexual characteristics, maintenance of reproductive organs, skeletal and muscle physiology, and feedback regulation of the hypothalamic-pituitary-gonadal axis.
Region-
SystemEndocrine System
Testosterone is the principal androgen produced by the testes and one of the major steroid hormones regulating male reproductive development and function. Within the testes, testosterone is synthesized primarily by Leydig cells, which lie in the interstitial tissue between the seminiferous tubules. Their steroidogenic activity is stimulated principally by luteinizing hormone (LH) from the anterior pituitary.
Testosterone is essential for differentiation and maintenance of the male internal reproductive tract, pubertal development, secondary sexual characteristics, sexual function, maintenance of spermatogenesis, and numerous systemic effects involving skeletal muscle, bone, skin, hematopoiesis, and the central nervous system.
Some testosterone acts directly through the androgen receptor, while some is converted in target tissues to dihydrotestosterone (DHT) or estradiol. These metabolites account for important tissue-specific actions of circulating testosterone.
Hormone Classification
Testosterone is a steroid hormone belonging to the androgen class. Like other steroid hormones, it is synthesized from cholesterol and is lipid soluble.
Its hydrophobic nature allows testosterone to cross cellular membranes and bind intracellular androgen receptors in responsive tissues.
Major Sources of Testosterone
| Source | Contribution |
|---|---|
| Testicular Leydig cells | Principal source of circulating testosterone in adult males |
| Adrenal cortex | Produces weaker androgen precursors that can contribute indirectly to androgen activity |
| Ovaries | Produce smaller amounts of testosterone and androgen precursors |
| Peripheral tissues | Can generate active androgens through conversion of circulating precursors |
Testicular Anatomy and Testosterone Production
The testes contain two major functional compartments: the seminiferous tubules, where spermatozoa develop, and the interstitial tissue, which contains testosterone-producing Leydig cells.
This anatomical arrangement allows endocrine testosterone production to occur immediately adjacent to the seminiferous tubules that require high local androgen concentrations for normal spermatogenesis.
Leydig Cells
Leydig cells, also called interstitial cells, are steroid-producing cells located in clusters within the connective tissue between seminiferous tubules.
They contain cellular machinery characteristic of steroidogenic cells, including abundant smooth endoplasmic reticulum and mitochondria specialized for steroid synthesis.
LH and Leydig Cells
Luteinizing hormone binds LH receptors on Leydig cells and stimulates testosterone synthesis. In males, LH has historically also been called interstitial cell-stimulating hormone because of this action.
LH receptor activation increases intracellular signaling that promotes cholesterol availability and steroidogenic enzyme activity.
Synthesis From Cholesterol
Testosterone synthesis begins with cholesterol. Cholesterol is transported into mitochondria, where the initial reactions of steroidogenesis occur.
Through a sequence of enzymatic reactions, cholesterol is converted into pregnenolone and subsequently through steroid intermediates into testosterone.
Basic Testosterone Synthesis
| Step | Process |
|---|---|
| 1 | LH binds receptors on Leydig cells |
| 2 | Intracellular steroidogenic signaling increases |
| 3 | Cholesterol is transported into mitochondria |
| 4 | Cholesterol is converted to pregnenolone |
| 5 | Steroidogenic enzymes generate androgen intermediates |
| 6 | Testosterone is produced and diffuses from the Leydig cell |
Steroidogenic Acute Regulatory Protein
The steroidogenic acute regulatory protein (StAR) facilitates movement of cholesterol into mitochondria, an important regulated step in steroid hormone synthesis.
LH signaling promotes steroidogenesis partly by increasing mechanisms involved in cholesterol transport and utilization.
Testosterone Secretion
Because steroid hormones are lipid soluble, testosterone is generally synthesized as needed rather than stored in large secretory vesicles. Once produced, it diffuses across Leydig-cell membranes.
Testosterone can enter nearby testicular tissue or the systemic circulation.
Intratesticular Testosterone
Testosterone concentrations within the testes are substantially higher than typical circulating concentrations. This high local androgen environment is important for normal development of germ cells within the seminiferous epithelium.
Maintenance of intratesticular testosterone depends substantially on LH stimulation of Leydig cells.
Testosterone and Sertoli Cells
Sertoli cells are located within the seminiferous tubules and provide structural and metabolic support to developing germ cells. They respond directly to FSH and also depend on androgen signaling for normal spermatogenic function.
Testosterone produced by adjacent Leydig cells therefore participates in paracrine regulation of the seminiferous epithelium.
Androgen-Binding Protein
Sertoli cells produce androgen-binding protein (ABP) under hormonal regulation, particularly involving FSH.
ABP binds testosterone and helps maintain a high androgen concentration within the seminiferous tubules, supporting spermatogenesis.
Testosterone and Spermatogenesis
Normal spermatogenesis requires both gonadotropin signaling and adequate intratesticular androgen activity. Testosterone acts through androgen receptors within the testicular environment to support the progression of germ-cell development.
Testosterone alone does not account for all regulation of spermatogenesis. FSH, Sertoli-cell factors, and local signaling pathways are also essential.
Seminiferous Tubules
The seminiferous tubules are highly coiled structures in which spermatozoa develop. Their epithelium contains Sertoli cells and successive generations of germ cells.
The close anatomical relationship between seminiferous tubules and interstitial Leydig cells facilitates hormonal coordination of sperm production.
Hypothalamic-Pituitary-Gonadal Axis
Testosterone production is regulated through the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates gonadotroph cells of the anterior pituitary.
The anterior pituitary subsequently releases LH and FSH, which act on different cellular compartments of the testes.
GnRH
Gonadotropin-releasing hormone is secreted by hypothalamic neurons in a pulsatile pattern. Pulsatile GnRH stimulates the anterior pituitary to synthesize and release LH and FSH.
The frequency and amplitude of GnRH pulses contribute to regulation of gonadotropin secretion.
LH
LH acts primarily on Leydig cells in the testes. Its major reproductive endocrine function in males is stimulation of testosterone production.
Changes in LH secretion therefore influence Leydig-cell steroidogenesis and circulating testosterone concentrations.
FSH
FSH acts primarily on Sertoli cells within the seminiferous tubules. It promotes functions required for normal spermatogenesis and stimulates production of regulatory proteins including inhibin B.
FSH and testosterone therefore act on complementary components of testicular function.
Basic Male HPG Axis
| Structure | Hormone or Target | Major Function |
|---|---|---|
| Hypothalamus | GnRH | Stimulates anterior pituitary gonadotrophs |
| Anterior pituitary | LH | Stimulates Leydig-cell testosterone synthesis |
| Anterior pituitary | FSH | Stimulates Sertoli-cell function |
| Leydig cells | Testosterone | Supports reproductive tissues, spermatogenesis, and systemic androgen actions |
| Sertoli cells | Inhibin B | Provides negative feedback on FSH secretion |
Negative Feedback
Testosterone participates in negative feedback regulation of the HPG axis. Increasing androgen activity reduces stimulatory signaling from the hypothalamus and anterior pituitary.
This feedback helps stabilize testicular testosterone production within the physiological endocrine system.
Feedback at the Hypothalamus
Androgenic and estrogenic signals derived from testosterone influence hypothalamic networks controlling GnRH secretion.
Some feedback effects of testosterone occur after its conversion to estradiol within the brain.
Feedback at the Pituitary
Testosterone and its metabolites also influence gonadotroph function at the anterior pituitary, contributing to regulation of LH secretion.
FSH regulation additionally depends strongly on inhibin B from Sertoli cells.
Testosterone and Inhibin B
Testosterone and inhibin B provide complementary feedback information about testicular function. Testosterone primarily reflects Leydig-cell steroidogenic activity, while inhibin B is produced principally by Sertoli cells.
Inhibin B selectively suppresses FSH secretion, whereas testosterone contributes more broadly to HPG-axis feedback.
Transport in Blood
Most circulating testosterone is bound to plasma proteins. Important binding proteins include sex hormone-binding globulin (SHBG) and albumin.
Only a relatively small fraction circulates as free testosterone.
Sex Hormone-Binding Globulin
SHBG is a plasma glycoprotein synthesized primarily by the liver. It binds testosterone with relatively high affinity.
Changes in SHBG concentration can alter the relationship between total testosterone and the free or bioavailable fractions.
Albumin-Bound Testosterone
A portion of circulating testosterone is weakly bound to albumin. Because this binding is less avid than SHBG binding, albumin-bound testosterone can dissociate relatively readily.
Assessment of androgen status may therefore consider total testosterone together with binding-protein physiology.
Free Testosterone
The free testosterone fraction is not bound to circulating proteins and is readily available for tissue uptake.
The proportion of free testosterone depends partly on circulating concentrations of SHBG and albumin.
Androgen Receptor
Testosterone exerts many of its effects through the androgen receptor (AR), an intracellular receptor belonging to the nuclear receptor superfamily.
Androgen receptors are expressed in reproductive tissues as well as skeletal muscle, bone, skin, brain, and numerous other organs.
Androgen Receptor Signaling
Testosterone enters target cells and binds the androgen receptor. Ligand binding alters receptor conformation and promotes interactions with chromatin and transcriptional regulatory proteins.
The resulting changes in gene expression produce tissue-specific androgen responses.
Androgen Response Elements
Activated androgen receptors can bind regulatory DNA sequences known as androgen response elements.
Recruitment of transcriptional cofactors then modifies expression of androgen-responsive genes.
Tissue-Specific Testosterone Action
Testosterone does not act identically in every tissue. Responses depend on androgen receptor expression, local enzyme activity, developmental stage, hormone concentration, and interactions with other signaling pathways.
Two enzymes are particularly important for modifying testosterone action: 5α-reductase and aromatase.
Dihydrotestosterone
Dihydrotestosterone (DHT) is a potent androgen produced from testosterone by the enzyme 5α-reductase.
DHT binds the androgen receptor with high potency and mediates several important androgen-dependent developmental and adult functions.
5α-Reductase
5α-reductase converts testosterone into DHT within selected target tissues.
This local conversion amplifies androgenic signaling in tissues where DHT has particularly important physiological roles.
Major DHT-Responsive Tissues
DHT has major roles in development and maintenance of structures including the prostate, external genitalia, and androgen-responsive skin and hair follicles.
The relative importance of testosterone and DHT therefore differs among androgen-sensitive tissues.
Testosterone Versus DHT
| Feature | Testosterone | DHT |
|---|---|---|
| Major source | Testicular Leydig cells | Peripheral conversion of testosterone |
| Key enzyme | Produced through testicular steroidogenesis | 5α-reductase |
| Receptor | Androgen receptor | Androgen receptor |
| Important developmental role | Male internal reproductive tract | Male external genitalia and prostate |
| Adult actions | Broad reproductive and systemic effects | Important effects in prostate, skin, and hair follicles |
Conversion to Estradiol
Testosterone can also be converted to estradiol by the enzyme aromatase. This occurs in several tissues, including adipose tissue, bone, and parts of the central nervous system.
Some physiological effects associated with testicular testosterone therefore depend on local estrogen formation.
Aromatase
Aromatase catalyzes conversion of androgen substrates into estrogens. In males, aromatization of testosterone provides an important source of estradiol.
This pathway is particularly relevant to bone physiology and neuroendocrine feedback.
Testosterone, DHT, and Estradiol
| Hormone | Formation | Important Roles |
|---|---|---|
| Testosterone | Primarily Leydig-cell secretion | Male reproductive development, spermatogenesis, muscle and systemic androgen effects |
| DHT | Testosterone converted by 5α-reductase | External genital development, prostate, skin, hair follicles |
| Estradiol | Testosterone converted by aromatase | Bone, feedback regulation, and other estrogen-dependent functions |
Testosterone During Fetal Development
Testicular androgen production is essential for normal male sexual differentiation during fetal development. Fetal Leydig cells produce testosterone under hormonal stimulation.
Testosterone and DHT subsequently act on different developing reproductive structures.
Development of the Male Internal Genital Tract
Testosterone promotes development and maintenance of the mesonephric (Wolffian) ducts in the male fetus.
These ducts give rise to important components of the internal male reproductive tract.
Wolffian Duct Derivatives
Under androgenic stimulation, the mesonephric ducts contribute to development of structures including the epididymis, ductus deferens, seminal vesicles, and ejaculatory ducts.
Testosterone is therefore directly important for differentiation of much of the internal duct system.
Development of External Genitalia
Development of the male external genitalia depends particularly on conversion of testosterone to DHT.
DHT drives masculinization of androgen-responsive external genital structures during fetal development.
Development of the Prostate
DHT is also particularly important for development of the prostate gland.
This illustrates why testosterone metabolism within target tissues is as important as circulating testosterone itself.
Testicular Descent
Androgen signaling contributes to the complex hormonal and anatomical processes involved in testicular descent.
Normal descent depends on multiple developmental factors and cannot be attributed to testosterone alone.
Testosterone During Childhood
After the early postnatal period, activity of the HPG axis becomes relatively low during much of childhood. Testicular testosterone production therefore remains comparatively low.
Reactivation of the reproductive axis at puberty produces a major increase in gonadotropin secretion and testicular androgen production.
Testosterone During Puberty
At puberty, increased pulsatile GnRH secretion stimulates greater LH and FSH release. LH stimulates Leydig cells, resulting in a marked increase in testosterone production.
Increasing androgen activity drives maturation of the male reproductive system and development of secondary sexual characteristics.
Growth of Male Reproductive Organs
Pubertal androgen exposure promotes growth and maturation of the penis, scrotum, prostate, seminal vesicles, epididymides, and other reproductive structures.
The testes also enlarge as seminiferous tubular development and spermatogenic activity increase.
Secondary Sexual Characteristics
Testosterone and DHT contribute to development of male secondary sexual characteristics during puberty.
These include characteristic patterns of body hair, changes in voice, increased muscle mass, skeletal changes, and alterations in skin and sebaceous gland activity.
Voice Changes
Androgen exposure promotes growth of the larynx and changes in the vocal apparatus during puberty.
These anatomical changes contribute to deepening of the voice.
Facial and Body Hair
Androgens regulate development of terminal hair in characteristic body regions. DHT has particularly important effects in some hair follicles.
Hair responses vary according to anatomical location, genetic factors, age, and local androgen metabolism.
Skin and Sebaceous Glands
Androgens stimulate sebaceous gland activity and influence skin physiology. Increased sebaceous secretion is one of the characteristic changes associated with puberty.
Testosterone and Skeletal Muscle
Testosterone has important anabolic effects on skeletal muscle. It promotes protein synthesis and contributes to increases in muscle mass and strength during male puberty and adulthood.
These effects vary according to androgen exposure, nutrition, physical activity, age, and other hormonal influences.
Testosterone and Bone
Testosterone contributes to skeletal growth, bone mass, and maintenance of adult bone. Some skeletal effects are mediated directly through androgen receptors, while others depend on conversion of testosterone to estradiol.
Both androgenic and estrogenic signaling are therefore important for male skeletal physiology.
Epiphyseal Maturation
Estradiol produced through aromatization of testosterone has an important role in maturation and eventual closure of the epiphyseal growth plates.
This demonstrates that estrogen signaling is physiologically important in the male skeleton.
Bone Mineral Density
Adequate gonadal steroid activity helps maintain bone mineral density. Reduced androgen production can therefore be associated with progressive loss of bone mass, partly through reduced androgenic and estrogenic signaling.
Testosterone and Hematopoiesis
Testosterone stimulates erythropoiesis and contributes to sex-related differences in hemoglobin and hematocrit after puberty.
Its hematological effects involve several mechanisms, including influences on erythropoietic regulation and iron metabolism.
Testosterone and Body Composition
Androgen activity influences the balance between lean body mass and adipose tissue. Testosterone promotes maintenance of skeletal muscle and contributes to characteristic patterns of body composition.
Changes in androgen status can therefore alter muscle mass and fat distribution.
Testosterone and Sexual Function
Testosterone contributes to sexual desire and supports the physiological environment required for normal male reproductive function.
Sexual function, however, depends on neurological, vascular, endocrine, psychological, and anatomical factors and is not controlled by testosterone alone.
Testosterone and Accessory Sex Glands
Androgens support growth and secretory function of male accessory reproductive glands, including the prostate and seminal vesicles.
These tissues contribute components of seminal fluid and depend on continued androgenic stimulation for normal structure and function.
Prostate
The prostate is highly androgen responsive. Local conversion of testosterone to DHT plays a particularly important role in prostatic androgen signaling.
Androgen activity contributes to development, maintenance, and secretory function of prostatic tissue.
Seminal Vesicles
The seminal vesicles are androgen-responsive accessory glands that produce a substantial portion of seminal fluid.
Testosterone supports their development and adult secretory function.
Epididymis and Ductus Deferens
The epididymis and ductus deferens are androgen-dependent structures derived largely from the mesonephric duct system.
Androgen signaling contributes to their development and maintenance.
Testosterone and the Brain
Androgen receptors are expressed within multiple regions of the central nervous system. Testosterone can influence neuroendocrine regulation, sexual behavior, mood-related processes, and other neural functions.
Some neural actions occur directly through androgen receptors, while others follow aromatization to estradiol.
Testosterone Secretion Pattern
Testosterone secretion is not constant. It reflects pulsatile LH stimulation and exhibits temporal variation.
In many adult males, circulating testosterone concentrations tend to be higher earlier in the day, although the magnitude of this diurnal pattern varies with age and other factors.
Testosterone Across the Lifespan
| Life Stage | General Pattern |
|---|---|
| Fetal life | Testicular androgen production supports male sexual differentiation |
| Early infancy | Transient activation of the HPG axis may increase testicular activity |
| Childhood | Relatively low gonadal androgen production |
| Puberty | Marked increase with HPG-axis activation |
| Adulthood | Maintained by pulsatile GnRH and LH stimulation |
| Aging | Average concentrations and HPG-axis characteristics may change progressively |
Testosterone Metabolism
Testosterone is metabolized in the liver and numerous peripheral tissues. It may be converted into active metabolites such as DHT and estradiol or into metabolites destined for elimination.
Conjugation reactions increase water solubility and facilitate excretion through urine and bile.
Testosterone Deficiency
Reduced testosterone activity can result from disorders affecting the testes, hypothalamus, pituitary, or other components of the reproductive endocrine system.
The physiological consequences depend on the age at which deficiency occurs, its severity, duration, and the underlying cause.
Primary Testicular Dysfunction
When the testes fail to produce adequate testosterone despite appropriate or increased gonadotropin stimulation, the disorder is considered primarily testicular in origin.
Loss of testicular negative feedback can result in increased LH concentrations.
Central Hypogonadism
Reduced GnRH or gonadotropin stimulation can produce inadequate Leydig-cell testosterone synthesis despite structurally intact testes.
This pattern reflects dysfunction at the hypothalamic or pituitary level rather than primary failure of Leydig cells.
Feedback Patterns in Testosterone Deficiency
| Site of Dysfunction | Testosterone | Typical Gonadotropin Relationship |
|---|---|---|
| Primary testicular dysfunction | Reduced | LH often elevated because negative feedback is reduced |
| Hypothalamic or pituitary dysfunction | Reduced | LH may be low or inappropriately normal |
Exogenous Androgens and Feedback
Androgen exposure from outside the normal testicular endocrine system can suppress hypothalamic and pituitary gonadotropin secretion through negative feedback.
Reduced LH lowers intratesticular testosterone production, while reduced gonadotropin support can impair spermatogenesis.
Circulating Versus Intratesticular Testosterone
Normal circulating androgen concentrations do not necessarily reproduce the very high local testosterone concentrations normally generated within the testes.
This distinction is important because spermatogenesis depends on the specialized intratesticular hormonal environment.
Androgen Insensitivity
Normal testosterone production cannot produce typical androgen-dependent effects if target tissues cannot respond appropriately to androgen signaling.
Alterations in androgen receptor function demonstrate the importance of the receptor, rather than hormone concentration alone, in determining biological androgen action.
5α-Reductase and Development
Impaired conversion of testosterone to DHT can selectively affect DHT-dependent tissues while leaving some testosterone-dependent processes relatively preserved.
This distinction demonstrates the different developmental roles of testosterone and its more potent metabolite DHT.
Testosterone in Females
Testosterone is also physiologically present in females. It is produced in smaller quantities by the ovaries and through peripheral conversion of androgen precursors, with additional contribution from adrenal androgen pathways.
Androgen signaling contributes to several aspects of normal female physiology, although circulating concentrations are substantially lower than typical adult male concentrations.
Testosterone and Estrogen in Males
Testosterone and estradiol should not be viewed as exclusively male and female hormones. Both have important physiological functions in males.
Aromatization of testosterone to estradiol contributes particularly to bone physiology and aspects of reproductive endocrine feedback.
Testosterone Versus DHT Versus Estradiol
| Feature | Testosterone | DHT | Estradiol |
|---|---|---|---|
| Relationship | Primary testicular androgen | Derived from testosterone | Derived partly from testosterone |
| Enzyme for conversion | Not applicable | 5α-reductase | Aromatase |
| Primary receptor | Androgen receptor | Androgen receptor | Estrogen receptors |
| Major examples of action | Internal reproductive tract, muscle, spermatogenic support | External genitalia, prostate, skin | Bone maturation and aspects of HPG feedback |
Testosterone and Male Reproductive Anatomy
| Structure | Relationship to Androgens |
|---|---|
| Leydig cells | Produce testosterone |
| Seminiferous tubules | Require high local androgen activity for normal spermatogenesis |
| Sertoli cells | Support germ cells and mediate important androgen-dependent spermatogenic functions |
| Epididymis | Androgen-dependent development and maintenance |
| Ductus deferens | Androgen-dependent internal reproductive duct |
| Seminal vesicles | Androgen-dependent accessory glands |
| Prostate | Strongly influenced by DHT |
| External genitalia | Fetal masculinization depends particularly on DHT |
Testosterone and Pubertal Changes
| System | Major Effect |
|---|---|
| Reproductive organs | Growth and functional maturation |
| Seminiferous tubules | Supports establishment of spermatogenesis with FSH and Sertoli-cell function |
| Muscle | Increased muscle mass and protein synthesis |
| Skeleton | Growth, increased bone mass, and eventual epiphyseal maturation |
| Larynx | Growth and deepening of voice |
| Hair follicles | Development of androgen-dependent terminal hair patterns |
| Skin | Increased sebaceous gland activity |
| Blood | Increased erythropoietic activity |
Key Features of Testosterone
| Feature | Key Point |
|---|---|
| Hormone class | Steroid androgen |
| Primary male source | Leydig cells of testes |
| Primary stimulatory hormone | LH |
| Precursor | Cholesterol |
| Principal receptor | Androgen receptor |
| Potent androgen metabolite | DHT |
| Enzyme producing DHT | 5α-reductase |
| Estrogenic metabolite | Estradiol |
| Enzyme producing estradiol | Aromatase |
| Major plasma binding proteins | SHBG and albumin |
| Feedback | Negative feedback within the HPG axis |
| Major reproductive role | Male differentiation, reproductive maintenance, and support of spermatogenesis |
Anatomical and Physiological Importance
Testosterone demonstrates the close relationship between testicular anatomy and endocrine function. Leydig cells occupy the interstitial spaces between seminiferous tubules, allowing testosterone produced under LH stimulation to act both systemically and within the local testicular environment. The high intratesticular androgen concentration created by Leydig-cell steroidogenesis is essential for normal spermatogenic function.
The testes contain complementary endocrine and spermatogenic compartments. LH acts on Leydig cells to stimulate testosterone production, while FSH acts primarily on Sertoli cells within the seminiferous tubules. Sertoli cells support developing germ cells and produce inhibin B, while testosterone provides essential androgenic signaling. These pathways integrate pituitary regulation with sperm production.
Testosterone also illustrates the importance of local hormone metabolism. Some tissues respond primarily to testosterone itself, while others convert testosterone to DHT through 5α-reductase. During fetal development, testosterone supports differentiation of the mesonephric duct derivatives, whereas DHT is particularly important for masculinization of the external genitalia and development of the prostate. In other tissues, aromatase converts testosterone to estradiol, which contributes importantly to bone physiology and endocrine feedback.
At puberty, activation of the HPG axis markedly increases testicular testosterone production. The resulting androgen exposure promotes maturation of the reproductive organs, establishment of the adult spermatogenic environment, development of secondary sexual characteristics, increased skeletal muscle mass, changes in the skeleton, and numerous other systemic effects.
Testosterone subsequently participates in negative feedback to the hypothalamus and anterior pituitary, creating a regulatory loop in which GnRH stimulates gonadotropins, LH stimulates Leydig-cell testosterone synthesis, and testicular hormones regulate further gonadotropin output. Testosterone is therefore both a major effector hormone and a feedback signal within the male reproductive endocrine axis.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026