Testosterone

Testosterone is the principal male sex hormone and a key androgen regulating the development, maintenance and function of reproductive tissues and secondary sexual characteristics. Although present in both sexes, it circulates at significantly higher levels in adult males and plays distinct roles across different stages of development. Beyond its reproductive functions, testosterone is critical for overall health, influencing mood, cognition, metabolism, cardiovascular function, and bone integrity. Alterations in testosterone levels—either deficiency or excess—can lead to wide-ranging physiological and behavioural effects.
Biochemically, testosterone is a steroid hormone of the androstane class, characterised by a ketone group at carbon-3 and a hydroxyl group at carbon-17. It is synthesised from cholesterol in a multi-step process and exerts its effects mainly through binding to the androgen receptor. The hormone is primarily produced by the testicles in males and to a lesser degree by the ovaries in females, with additional contributions from adrenal glands.

Biological Effects

Testosterone influences a broad array of developmental and physiological processes. Its actions are typically categorised into anabolic and androgenic effects, though both aspects overlap considerably.
• Anabolic effects include increased protein synthesis, muscle growth, enhanced bone density and strength, and stimulation of linear growth.• Androgenic (virilising) effects include differentiation of male reproductive structures, deepening of the voice, growth of facial and body hair, and other secondary sexual characteristics.
Testosterone may act directly on tissues or be converted to other active hormones. Conversion to dihydrotestosterone (DHT) via 5α-reductase produces a more potent androgen with stronger receptor affinity, important in the development of external genitalia and certain hair patterns. Conversion to estradiol (E2) by aromatase is essential for bone maturation and influences behavioural and cognitive processes.

Effects Across Developmental Stages

Before Birth

Prenatal testosterone operates during two key developmental windows. Between 4 and 6 weeks of gestation, it contributes to virilisation of male genital structures, although DHT plays a more substantial role. During the second trimester, testosterone—alongside anti-Müllerian hormone—promotes growth of the Wolffian ducts while Müllerian ducts regress, establishing male reproductive anatomy.
Prenatal androgens are also associated with sex-typed behaviours and may influence later preferences and cognitive patterns. Research on congenital adrenal hyperplasia in females highlights how increased prenatal androgen exposure relates to male-typical childhood play and variations in adult gender-related interests.

Early Infancy

Male infants experience a temporary postnatal surge in testosterone during the first months of life. Although the precise function remains uncertain, this surge may contribute to early brain masculinisation. In this period, estradiol derived from testosterone can cross the blood–brain barrier in males due to the absence of fetoprotein binding, a mechanism that protects the female brain from masculinising effects.

Before Puberty

Rising androgen levels in both sexes during later childhood produce subtle changes: adult-type body odour, increased skin oiliness, the onset of acne, growth of pubic and axillary hair, and initial acceleration of bone development.

Puberty

Sustained elevation of testosterone beyond typical adult female levels triggers the full spectrum of pubertal changes in males and, under abnormal hormonal conditions, in females. These include:
• enlargement of the penis or clitoris• growth of spermatogenic tissue and onset of fertility• deepening of the voice and development of the laryngeal prominence• broadening of the shoulders and expansion of the rib cage• increased muscle mass and strength• redistribution of body fat and growth of facial, chest and body hair• potential development of androgenetic alopecia
Completion of bone growth and closure of epiphyses occur indirectly through estradiol metabolites and therefore proceed more slowly in males.

Adult Life

In adults, testosterone supports normal sperm development by activating genes in Sertoli cells. It enhances muscle protein synthesis, influences platelet aggregation through thromboxane A2 receptor regulation, and contributes to behavioural responses such as dominance-related effects involving the hypothalamic–pituitary–adrenal axis.
Testosterone also plays a key role in sexual motivation, mood regulation, and metabolic processes. Brain differences between sexes—such as certain structural and functional variations—may partly reflect lifelong androgen exposure, although environmental factors also contribute.

Health Effects

Testosterone levels influence numerous aspects of wellbeing. Insufficient testosterone in males may result in fatigue, increased adipose tissue, reduced libido, infertility, diminished bone density, anxiety, and depressive symptoms. Excess testosterone can contribute to hyperandrogenism, increased cardiovascular risk, male pattern baldness, and accelerated progression of existing prostate cancer in individuals following testosterone deprivation therapy. Current evidence does not indicate that physiological testosterone levels raise the baseline risk of developing prostate cancer.
In both sexes, balanced testosterone levels support cognitive performance, healthy metabolic function, stable mood, and overall vitality. Declines in later adulthood, sometimes described as andropause, have led to medical interest in testosterone replacement therapies, though these must be carefully monitored.

Medical and Non-medical Use

Testosterone is used therapeutically to treat male hypogonadism and certain breast cancers. In older men, supplementation is sometimes prescribed to counteract deficiency symptoms. However, testosterone and its analogues are also used illicitly as performance-enhancing substances in sport. Due to their anabolic properties and potential health risks, these agents are classified by the World Anti-Doping Agency as prohibited anabolic substances.