TESTOSTERONE: Physiology, Function and Systemic Effects in All Sexes
Testosterone is the principal androgen in humans and plays vital roles across reproductive, metabolic, neurological and musculoskeletal systems. While traditionally associated with male physiology, testosterone is also essential in females—supporting ovarian function, libido, bone health and overall endocrine balance. Produced primarily by the testes in males and in smaller amounts by the ovaries and adrenal glands in females, testosterone influences sexual development, reproductive capacity and multiple systemic processes. A clear understanding of its mechanisms and effects across sexes provides important insight into fertility, puberty, endocrine disorders and general health.
What You Need to Know
Testosterone is a steroid hormone with essential reproductive and systemic roles in all sexes. Although circulating concentrations differ markedly between males and females, testosterone influences cellular function, metabolism, musculoskeletal health, and neuroendocrine signalling across the lifespan. Its effects depend not only on serum concentration but also on tissue sensitivity, local conversion to other hormones, and availability of free hormone.
In males, testosterone is synthesised primarily by Leydig cells in the testes under stimulation from luteinising hormone. High intratesticular testosterone concentrations are required for normal spermatogenesis, while circulating testosterone supports development and maintenance of male reproductive organs, secondary sexual characteristics, libido, muscle mass, and bone density. Testosterone production follows a circadian rhythm, with highest levels in the early morning, and declines gradually with age due to reduced Leydig cell responsiveness.
Key physiological actions of testosterone include:
Support of spermatogenesis, through high local concentrations within the testes
Development and maintenance of secondary sexual characteristics, including body hair distribution and voice depth
Anabolic effects, promoting muscle protein synthesis and bone mineralisation
Regulation of libido, mood, and energy levels, via central nervous system actions
In females, testosterone is produced in smaller amounts by the ovarian theca cells and the adrenal cortex. It serves as a crucial precursor for estrogen synthesis through aromatisation within granulosa cells, linking androgen and estrogen pathways. Despite lower circulating levels, testosterone contributes to libido, follicular development, bone integrity, and metabolic regulation. Excess or deficiency can therefore have significant reproductive and systemic consequences.
Most circulating testosterone is bound to sex hormone–binding globulin or albumin, leaving only a small free fraction that is biologically active. Changes in SHBG levels, influenced by factors such as age, body composition, thyroid status, and estrogen exposure, can alter testosterone availability without changing total testosterone concentrations. Understanding this distinction is essential when interpreting hormone results and assessing clinical relevance.
Beyond the Basics
Biosynthesis and Regulation
Testosterone is synthesised from cholesterol through a multi-step enzymatic pathway. In males, luteinising hormone stimulates Leydig cells to convert cholesterol into testosterone, generating extremely high local concentrations within the testes. These intratesticular levels are essential for spermatogenesis and far exceed circulating concentrations. Testosterone then diffuses into the seminiferous tubules, where it acts alongside follicle-stimulating hormone and Sertoli cells to support germ cell development.
In females, testosterone is produced in smaller amounts by ovarian theca cells and the adrenal cortex. Within the ovary, it serves as a substrate for aromatase in granulosa cells, allowing conversion into estrogen. This close biochemical relationship illustrates how androgen and estrogen pathways are tightly linked rather than functionally separate.
Testosterone secretion is regulated through negative feedback at both the hypothalamus and pituitary. Rising testosterone suppresses gonadotropin-releasing hormone and luteinising hormone release, stabilising androgen levels and preventing excessive hormone production.
Androgen Receptors and Mechanism of Action
Testosterone exerts its effects by binding to intracellular androgen receptors, which then translocate to the nucleus and modify gene transcription. In certain tissues, testosterone is converted to dihydrotestosterone by the enzyme 5α-reductase. Dihydrotestosterone has greater receptor affinity and is responsible for many tissue-specific androgenic effects, including development of external genitalia, prostate growth, facial hair distribution and some skin changes.
Androgen receptors are widely distributed throughout the body, including in reproductive organs, bone, muscle, brain and cardiovascular tissue. This broad receptor expression explains why testosterone influences not only reproduction, but also musculoskeletal integrity, metabolism, mood and cognitive function.
Physiological Effects in Males
During puberty, testosterone drives maturation of the male reproductive system and development of secondary sexual characteristics. These include enlargement of the penis, scrotum and prostate, deepening of the voice due to laryngeal growth, increased muscle mass, changes in body composition, and growth of facial and body hair. Libido rises as central nervous system sensitivity to androgens increases.
In adulthood, testosterone maintains spermatogenesis, erectile physiology, red blood cell production, bone mineral density and metabolic balance. Its actions within the brain contribute to motivation, mood regulation and cognitive performance. Deficiency therefore produces both physical and neuropsychological effects rather than isolated reproductive symptoms.
Physiological Effects in Females
Although circulating testosterone levels are much lower in females, physiological sensitivity to androgens remains high. Testosterone contributes to sexual desire and arousal, supports energy levels and motivation, and plays a role in follicular development by providing substrate for estrogen synthesis. It also supports musculoskeletal health, maintenance of lean body mass, cognitive function and overall metabolic stability.
In premenopausal females, the ovaries and adrenal glands contribute roughly equal amounts of circulating testosterone. After menopause, ovarian production declines, and adrenal-derived androgens become increasingly important for maintaining baseline androgen activity.
Age-Related Changes
In males, testosterone levels decline gradually with age due to reduced Leydig cell responsiveness rather than abrupt endocrine failure. This gradual reduction may contribute to decreased libido, fatigue, reduced muscle mass, increased visceral fat and declining bone density. The changes are variable and strongly influenced by overall health, body composition and chronic disease.
In females, testosterone peaks in early adulthood and slowly declines across the lifespan, with a more noticeable reduction after menopause. These changes may contribute to alterations in sexual function, musculoskeletal strength and energy levels. Together, age-related androgen changes in both sexes highlight testosterone’s role as a lifelong regulator of systemic health rather than a hormone limited to reproductive years.
Clinical Connections
Abnormal testosterone levels have wide-ranging clinical implications because testosterone influences reproductive capacity, metabolic health, musculoskeletal integrity, and psychological wellbeing. Symptoms are often gradual and nonspecific, which means testosterone disorders may be overlooked unless actively considered during assessment.
In males, low testosterone, or hypogonadism, may arise from primary testicular failure, hypothalamic or pituitary dysfunction, chronic systemic illness, ageing, or suppression from exogenous androgens. Reduced androgen signalling affects both reproductive and non-reproductive tissues, explaining why symptoms often extend beyond sexual function alone.
Clinical features of testosterone deficiency in males commonly include:
Reduced libido and erectile dysfunction, reflecting impaired androgen signalling
Infertility, due to suppressed spermatogenesis
Fatigue and low energy, linked to central nervous system effects
Loss of muscle mass and strength, with increased fat mass
Anaemia and reduced bone density, increasing fracture risk
Testosterone therapy may improve symptoms in carefully selected patients, but it suppresses gonadotropin release and intratesticular testosterone production. As a result, exogenous testosterone can significantly impair or abolish spermatogenesis, making it inappropriate for individuals seeking fertility unless alternative strategies are used.
In females, interpretation of testosterone levels is more complex because normal concentrations are much lower and reference ranges are narrower. Low testosterone may contribute to reduced libido, fatigue, low mood, and decreased bone density, but diagnosis requires careful clinical correlation rather than reliance on laboratory values alone.
Excess testosterone in females is clinically significant and more readily apparent. Conditions such as polycystic ovary syndrome or androgen-secreting tumours may present with hirsutism, acne, scalp hair thinning, menstrual irregularity, and infertility. These features reflect androgen effects on skin, hair follicles, and ovarian function rather than isolated hormonal imbalance.
In males, excess testosterone most commonly results from anabolic steroid use rather than endogenous overproduction. Supraphysiological (when a substance is higher than normal) androgen levels suppress the hypothalamic–pituitary–gonadal axis (a neuroendocrine system that regulates reproduction), leading to reduced LH and FSH, testicular atrophy, impaired sperm production, and infertility. Psychological effects, cardiovascular risk, and adverse lipid changes may also occur.
Concept Check
How does LH regulate testosterone synthesis in males, and which cells are primarily responsible?
Why is testosterone essential for spermatogenesis even though it circulates mostly in a bound form?
What roles does testosterone play in female reproductive physiology?
How does the conversion of testosterone to DHT influence tissue-specific androgen effects?
What clinical signs might indicate excess testosterone in females, and why?