Testosterone
Testosterone is a steroid hormone derived from cholesterol and classified as the primary male sex hormone and an androgen, playing a crucial role in the development and maintenance of male reproductive tissues such as the testes and prostate, as well as secondary sexual characteristics like increased muscle mass, body hair, and a deeper voice.[1][2] Produced mainly by Leydig cells in the testes in males and in smaller quantities by the ovaries and adrenal glands in females, testosterone is synthesized from precursors like dehydroepiandrosterone (DHEA) and androstenedione, with a portion converted to the more potent dihydrotestosterone (DHT) by the enzyme 5-alpha-reductase.[1][3] In physiological terms, testosterone drives fetal sex differentiation by promoting the development of male genitalia and suppressing female structures, while during puberty it triggers growth spurts, genital maturation, and libido enhancement in both sexes.[2][1] Beyond reproduction, it supports spermatogenesis, bone density, red blood cell production (erythropoiesis), and fat distribution, with levels peaking in early adulthood and gradually declining thereafter—typically by about 1% per year after age 30.[1][3] In females, where circulating levels are much lower (usually under 40 ng/dL compared to 193–824 ng/dL in adult males), testosterone contributes to ovarian function, libido, and muscle strength, often being aromatized into estradiol for estrogenic effects.[2][3] Regulation of testosterone occurs through the hypothalamic-pituitary-gonadal (HPG) axis, where gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the pituitary gland to release luteinizing hormone (LH), which in turn prompts gonadal production; negative feedback from high testosterone levels inhibits this pathway to maintain homeostasis.[1][3] Circadian rhythms influence secretion, with peak levels in the morning, and disruptions can lead to imbalances: low testosterone (hypogonadism) in males may cause fatigue, reduced fertility, erectile dysfunction, and loss of muscle mass, while excess in females is associated with polycystic ovary syndrome (PCOS), manifesting as hirsutism, acne, and irregular menstruation.[2][1] Medically, testosterone replacement therapy is used to treat deficiencies, though it carries risks like cardiovascular events and prostate issues, underscoring its potent anabolic-androgenic properties.[3][1]Biochemistry
Biosynthesis
Testosterone is primarily synthesized in the Leydig cells of the testes, which constitute about 5% of the testicular volume and are located in the interstitial space.[4] The process begins with cholesterol as the precursor and is regulated by luteinizing hormone (LH) from the pituitary gland, which binds to the LH/choriogonadotropin receptor (LHCGR) on Leydig cells, activating adenylate cyclase to increase cAMP levels and promote cholesterol transport into mitochondria.[4] This transport is mediated by the steroidogenic acute regulatory protein (StAR), a critical step in steroidogenesis.[5] In adult men, the testes produce approximately 3–10 mg of testosterone per day, accounting for over 95% of circulating testosterone, with minor contributions from the adrenal glands and peripheral conversion.[4] The biosynthesis pathway involves a series of enzymatic reactions divided between mitochondria (initial steps) and the smooth endoplasmic reticulum (subsequent steps). The rate-limiting step is the conversion of cholesterol to pregnenolone by the mitochondrial enzyme CYP11A1 (cholesterol side-chain cleavage enzyme), which cleaves the side chain of cholesterol.[4] From pregnenolone, testosterone is produced via two canonical pathways: the Δ5 pathway (preferred in humans) and the Δ4 pathway. In the Δ5 pathway, pregnenolone is hydroxylated to 17α-hydroxypregnenolone by CYP17A1 (17α-hydroxylase activity), followed by side-chain cleavage to dehydroepiandrosterone (DHEA) via CYP17A1 (17,20-lyase activity); DHEA is then converted to androstenedione by 3β-hydroxysteroid dehydrogenase (3β-HSD), and finally, androstenedione is reduced to testosterone by 17β-hydroxysteroid dehydrogenase type 3 (HSD17B3).[5][6] The Δ4 pathway diverges earlier, with pregnenolone isomerized to progesterone by 3β-HSD, then hydroxylated to 17α-hydroxyprogesterone by CYP17A1, cleaved to androstenedione by CYP17A1 lyase, and reduced to testosterone by HSD17B3.[6][4] An alternative "backdoor" pathway contributes to androgen production, particularly during fetal development, bypassing testosterone as an intermediate to directly form dihydrotestosterone (DHT) from precursors like 17α-hydroxyprogesterone via enzymes including 5α-reductase types 1 and 2 (SRD5A1/2).[5] This pathway is prominent in species like the tammar wallaby and plays a role in human fetal masculinization, though its contribution in adults is less dominant.[5] Key enzymes in testosterone biosynthesis are summarized below:| Enzyme | Function | Location |
|---|---|---|
| StAR | Facilitates cholesterol transport into mitochondria | Mitochondria |
| CYP11A1 | Converts cholesterol to pregnenolone (rate-limiting) | Mitochondria |
| CYP17A1 | 17α-hydroxylation and 17,20-lyase activity to form DHEA or androstenedione | Endoplasmic reticulum |
| 3β-HSD | Isomerizes Δ5 steroids (e.g., DHEA) to Δ4 steroids (e.g., androstenedione) | Endoplasmic reticulum |
| HSD17B3 | Reduces androstenedione to testosterone | Endoplasmic reticulum |
Metabolism
Testosterone undergoes extensive catabolism in humans primarily through hepatic and peripheral tissue metabolism, involving reduction, oxidation, hydroxylation, and conjugation to facilitate inactivation and excretion. The liver serves as the main site for these transformations, where testosterone is converted into polar, water-soluble metabolites that are readily eliminated. Approximately 90% of testosterone metabolites are excreted in urine, with the remainder via feces.[7] A primary catabolic route involves reduction pathways. Testosterone is reduced at the 5α position by steroid 5α-reductase enzymes (SRD5A1 and SRD5A2) to form the more potent dihydrotestosterone (DHT), but further 3α-reduction by aldo-keto reductase 1C (AKR1C) subfamily enzymes inactivates DHT to 5α-androstan-3α,17β-diol and ultimately to androsterone, a 17-ketosteroid. Alternatively, 5β-reduction by AKR1D1 yields 5β-dihydrotestosterone, which is then 3α-reduced to etiocholanolone, another key 17-ketosteroid. These 5α- and 5β-reduced metabolites represent major inactive end products, with androsterone and etiocholanolone accounting for a significant portion of urinary 17-ketosteroid output.[7][8] Oxidation pathways contribute to inactivation by converting testosterone's 17β-hydroxy group to a 17-keto group. This is catalyzed by 17β-hydroxysteroid dehydrogenase enzymes (HSD17B2 and HSD17B4), transforming testosterone to androstenedione, which is excreted at a ratio of about 1:10 compared to 17β-hydroxy forms in urine. Additional hydroxylation at positions like C11β, mediated by cytochrome P450 enzymes such as CYP3A4, CYP2C9, and CYP2C19, introduces hydroxyl groups to enhance polarity.[7] Prior to excretion, most metabolites undergo conjugation to increase solubility. Glucuronidation, primarily by UDP-glucuronosyltransferase enzymes (UGTs) including UGT2B7, UGT2B15, and UGT2B17, forms conjugates like androsterone glucuronide and etiocholanolone glucuronide, which predominate in urine. Sulfation by sulfotransferases and, to a lesser extent, cysteine conjugation also occur, particularly for epitestosterone and other derivatives. About 50% of circulating testosterone is directly conjugated to testosterone glucuronide or sulfate in the liver before further breakdown. These conjugated 17-ketosteroids are the principal urinary markers of androgen metabolism.[7][8] The backdoor pathway represents an alternative catabolic route, particularly active during fetal development, where precursors like pregnenolone are reduced to 5α-dihydro metabolites, leading to DHT and androsterone without passing through free testosterone. This pathway underscores tissue-specific variations in metabolism, with overall clearance rates influenced by age, sex, and health status.[7]Levels
Testosterone levels in the blood are typically measured as total testosterone, which includes both free (unbound) and bound forms (primarily to sex hormone-binding globulin [SHBG] and albumin), or as free testosterone, the unbound fraction biologically active at target tissues.[9] Total testosterone is the most commonly assessed parameter in clinical practice, with reference ranges established using standardized methods like liquid chromatography-tandem mass spectrometry (LC-MS/MS) for accuracy.[9] In adult men, harmonized reference ranges for total serum testosterone in healthy, nonobese individuals aged 19–39 years are 264–916 ng/dL (9.2–31.8 nmol/L), based on the 2.5th to 97.5th percentiles from large cohort studies calibrated to CDC standards.[9] These levels decline progressively with age; for example, in men aged 70–79 years, the range narrows to approximately 218–926 ng/dL (7.6–32.1 nmol/L), reflecting a 1–2% annual decrease starting around age 30.[9] Approximately 20% of men over 60 years have total testosterone below the young adult normal range, rising to 50% in those over 80.[10] Free testosterone levels in men follow a similar pattern, with normative ranges of 66–309 pg/mL (229–1072 pmol/L) across adulthood, though higher in younger men at 120–368 pg/mL (415–1274 pmol/L).[11] In adult women, total serum testosterone levels are substantially lower, with reference ranges of 15–46 ng/dL (0.52–1.60 nmol/L) for normally cycling premenopausal individuals around age 30, derived from the 5th to 95th percentiles in validated immunoassays.[12] Free testosterone in women ranges from 1.2–6.4 pg/mL (4.2–22.2 pmol/L) under similar conditions.[12] Levels in women also decline with age, by about 35% from ages 53–60 to 60–64, paralleling reductions in ovarian function, though postmenopausal values remain within 7.1–49.8 ng/dL for the 10th–90th percentiles in adults over 20.[13][14] Testosterone exhibits a diurnal rhythm in both sexes, with peak concentrations in the early morning and nadir in the late afternoon or evening. In men aged 30–40 years, levels are 20–25% lower at 4:00 PM compared to 8:00 AM, a variation that diminishes to about 10% by age 70.[15] Women show a similar pattern, with significantly higher morning levels, though the amplitude is generally smaller.[16] Clinical measurements are thus recommended in the morning to standardize assessments.[15]| Parameter | Men (19–39 years) | Women (Premenopausal, ~30 years) | Notes |
|---|---|---|---|
| Total Testosterone | 264–916 ng/dL | 15–46 ng/dL | Harmonized LC-MS/MS for men; immunoassay for women. Declines with age in both.[9][12] |
| Free Testosterone | 120–368 pg/mL | 1.2–6.4 pg/mL | Biologically active fraction; ~2% of total in men, ~1–2% in women.[11][12] |
Biological Activity
Steroid Hormone Activity
Testosterone functions primarily as an androgenic steroid hormone, exerting its effects by binding to the androgen receptor (AR), a nuclear receptor that mediates genomic signaling in target tissues such as muscle, bone, and reproductive organs. Upon entering the cell, testosterone diffuses across the plasma membrane and binds to the AR in the cytoplasm, inducing a conformational change that releases heat shock proteins and allows the hormone-receptor complex to dimerize and translocate to the nucleus. There, the complex binds to androgen response elements (AREs) in the DNA, recruiting coactivators and corepressors to modulate gene transcription, ultimately influencing protein synthesis and cellular function. This classical genomic pathway accounts for many of testosterone's long-term effects, including muscle hypertrophy and spermatogenesis, with response times typically ranging from hours to days. In addition to genomic actions, testosterone exhibits non-genomic steroid hormone activity through rapid signaling mechanisms that do not involve direct gene transcription. These effects occur within seconds to minutes and are mediated by membrane-associated AR variants or interactions with other receptors, such as G-protein coupled receptors, leading to activation of kinase pathways like MAPK/ERK and PI3K/Akt. For instance, in vascular smooth muscle cells, testosterone rapidly induces vasodilation via calcium influx and nitric oxide production, independent of AR nuclear translocation. Non-genomic actions also contribute to acute metabolic responses, such as increased glucose uptake in skeletal muscle. Testosterone's steroid hormone activity is further modulated by its conversion to more potent metabolites: 5α-dihydrotestosterone (DHT) via 5α-reductase, which binds AR with higher affinity and drives prostate growth and hair follicle effects, and estradiol via aromatase, enabling estrogen receptor activation in tissues like bone and brain. These transformations amplify or diversify testosterone's hormonal signaling, with tissue-specific expression of enzymes determining local androgen and estrogen bioavailability. Disruptions in these pathways, such as AR mutations, can lead to androgen insensitivity syndromes, underscoring the receptor's central role in steroid hormone mediation.Neurosteroid Activity
Testosterone serves as a neurosteroid, synthesized de novo in the brain or derived from peripheral sources, where it modulates neuronal function through both genomic and non-genomic mechanisms. Unlike its classical endocrine roles, neurosteroid activity involves rapid signaling at membrane receptors, influencing excitability, plasticity, and neurotransmitter systems such as GABAergic and glutamatergic pathways. This local brain action allows testosterone to exert direct effects on cognition, emotion, and behavior, distinct from systemic hormone influences.[17] Within the central nervous system, testosterone undergoes metabolism to active neurosteroids via key enzymatic pathways. Aromatization by cytochrome P450 aromatase converts it to 17β-estradiol, which supports synaptic plasticity and neuroprotection. Alternatively, 5α-reductase reduces testosterone to dihydrotestosterone (DHT), which can be further metabolized by 3α-hydroxysteroid dehydrogenase to 3α-androstanediol (3α-diol), a potent positive allosteric modulator of GABA_A receptors that enhances inhibitory neurotransmission and reduces neuronal excitability. These metabolites are produced on demand in regions like the hippocampus, cortex, and midbrain, enabling localized regulation of neural circuits.[18] Testosterone's neurosteroid effects promote adult neurogenesis, particularly in the dentate gyrus of the hippocampus, where it enhances the survival of newly generated neurons via androgen receptor activation and downstream signaling involving brain-derived neurotrophic factor (BDNF) and mitogen-activated protein kinase (MAPK) pathways, without significantly altering cell proliferation. This contributes to improved spatial memory and cognitive performance in males, as demonstrated in rodent models where testosterone supplementation restores hippocampal-dependent tasks impaired by gonadectomy. Additionally, it modulates brain wave activity, increasing power in delta, theta, beta, and gamma bands while restoring alpha waves, which correlates with enhanced motor function and reduced anxiety-like behaviors.[19][20] In terms of affective and motivational processes, testosterone-derived neurosteroids like 3α-diol facilitate social and sexual behaviors by acting in the ventral tegmental area, promoting dopamine release and reward-related motivation independent of classical nuclear receptors. They also exhibit anxiolytic and antidepressant effects, reducing depression-like symptoms in animal models through GABA_A modulation in the hippocampus and amygdala. Neuroprotective roles are evident in mitigating oxidative stress and apoptosis; for instance, 3α-diol inhibits ERK phosphorylation and caspase-3 activation against hydrogen peroxide-induced toxicity in neuronal cell lines, with implications for sex differences in neurodegenerative diseases like Alzheimer's. These actions underscore testosterone's broader impact on brain health, though levels decline with aging, potentially exacerbating cognitive deficits.[17][18][20]Free Testosterone
Free testosterone (FT) is the unbound fraction of circulating testosterone that is not attached to carrier proteins such as sex hormone-binding globulin (SHBG) or albumin, representing the biologically active form capable of entering target cells to exert androgenic effects.[21] This unbound portion constitutes approximately 1-4% of total testosterone levels in the blood, with the remainder primarily bound to SHBG (about 45-60%) and albumin (about 35-50%).[22] The free form's availability is crucial for testosterone's role in promoting muscle protein synthesis, erythropoiesis, and libido through binding to intracellular androgen receptors.[23] In physiological terms, free testosterone serves as the primary mediator of testosterone's steroid hormone activity, diffusing freely across cell membranes to activate gene transcription in responsive tissues like skeletal muscle, bone, and the prostate.[21] Unlike bound testosterone, which acts as a reservoir, FT directly influences metabolic processes, including glucose uptake and fat distribution, and its levels correlate more closely with clinical symptoms of androgen deficiency than total testosterone in conditions altering SHBG binding, such as insulin resistance or liver disease.[24] For instance, lower FT is associated with increased insulin levels and higher risk of metabolic syndrome in men.[24] Measurement of free testosterone is essential for accurate assessment of androgen status, particularly when total testosterone measurements may be misleading due to variations in binding proteins.[21] The gold standard method is equilibrium dialysis combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS), which separates unbound testosterone from protein-bound forms under physiological conditions.[23] An alternative, widely used approach is calculation based on total testosterone, SHBG, and albumin concentrations using validated equations, such as that proposed by Vermeulen et al., which provides a reliable estimate correlating well with dialysis results.[25] Direct analog immunoassays, though faster, often underestimate FT and are less recommended.[21] Reference ranges for free testosterone vary by age, sex, and assay method, but typical values for adult men aged 20-40 years are 5-21 ng/dL (or 50-210 pg/mL), declining gradually with age to about 4-15 ng/dL in men over 60.[23] In adult women, levels are substantially lower, ranging from 0.1-1.1 ng/dL (1-10 pg/mL), reflecting their role in maintaining ovarian function and mild androgenic effects.[23] These measurements are particularly valuable in diagnosing hypogonadism or hyperandrogenism, guiding therapeutic decisions like testosterone replacement therapy.[21]Physiological Effects
Prenatal and Early Development
Testosterone plays a pivotal role in the prenatal sexual differentiation of male fetuses, beginning around weeks 6–7 of gestation when the gonads differentiate into testes under the influence of the SRY gene.[26] In male embryos, Leydig cells in the testes start producing testosterone by week 9, with levels peaking between weeks 14–17, driving the masculinization of internal and external reproductive structures.[26] This hormone stabilizes the Wolffian ducts, which develop from around day 24 and differentiate into the epididymis, vas deferens, and seminal vesicles by weeks 9–13, through binding to androgen receptors that upregulate male-specific genes.[26] Concurrently, Sertoli cells secrete anti-Müllerian hormone (AMH) from weeks 7–9, inducing regression of the Müllerian ducts by weeks 8–12 via apoptosis pathways involving TGF-β and WNT/β-catenin signaling, preventing the formation of female internal structures like the uterus and fallopian tubes.[26] For external genitalia, testosterone is converted to dihydrotestosterone (DHT) by 5α-reductase type 2, which masculinizes the genital tubercle into the penis and fuses the labioscrotal folds into the scrotum between weeks 9–14.[26] This process includes lengthening of the anogenital distance by week 9 and closure of the urethral groove by week 14, with prostatic buds emerging around week 10 to form the prostate.[26] In the absence of testosterone, as in female fetuses, default development leads to female-typical genitalia, including the clitoris and labia. Disruptions, such as in congenital adrenal hyperplasia (CAH), where excess prenatal androgens affect females, result in masculinized genitalia, highlighting testosterone's organizational effects.[27] Beyond physical structures, prenatal testosterone influences brain sexual differentiation during a critical window from weeks 8–24, organizing neural circuits that affect later behaviors by binding to androgen receptors and being aromatized to estrogen in certain regions.[28] In humans, elevated fetal testosterone levels, measured via amniotic fluid between weeks 11–21, correlate with increased male-typical play behaviors in childhood, such as preferences for toy vehicles over dolls, in both boys (r = 0.20, p < 0.05) and girls (r = 0.42, p < 0.001), as shown in a longitudinal study of 212 children.[29] Girls with CAH, exposed to high prenatal androgens, exhibit more male-typical toy preferences, activity levels, and aggression across multiple studies, supporting an enduring organizational role for testosterone in behavioral sexual differentiation.[28] Additionally, prenatal testosterone exposure is linked to later sexual orientation, with CAH women showing reduced rates of exclusive heterosexuality (approximately 41% non-heterosexual vs. 5% in controls).[27] In early postnatal development, a "mini-puberty" surge in testosterone occurs in male infants during months 1–6, peaking around month 1, which may further consolidate prenatal effects on brain and body maturation, though its precise influences on human gender development remain under investigation.[28] Overall, these prenatal and early exposures establish foundational sex differences, with long-term implications for reproductive function, gender identity, and social behaviors, as evidenced by conditions like CAH and androgen insensitivity syndrome.[28]Pubertal and Adult Development
During puberty, testosterone levels in males surge dramatically, typically increasing 30-fold from childhood levels, driven by the activation of the hypothalamic-pituitary-gonadal axis that stimulates Leydig cells in the testes to produce the hormone.[1] This surge, beginning around ages 11 to 15, triggers a growth spurt by promoting linear bone growth through epiphyseal plate activity before eventually contributing to their closure, resulting in increased height and the development of a more masculine skeletal frame.[2] Testosterone also deepens the voice by enlarging the larynx and induces the growth of facial, axillary, and pubic hair through stimulation of hair follicles.[30] Additionally, it enlarges the penis, testes, and prostate gland, while initiating spermatogenesis and boosting libido, marking the onset of reproductive maturity.[1] In females, testosterone, produced in smaller amounts by the ovaries and adrenal glands, contributes to pubertal changes such as pubic and axillary hair growth and mild increases in muscle mass, though its effects are less pronounced and often mediated through conversion to estradiol.[2] Overall, these pubertal transformations establish secondary sexual characteristics that define male physiology, with normal testosterone levels rising to 102-1010 ng/dL by ages 16-17 in males.[2] In adulthood, testosterone maintains these secondary sexual characteristics and supports ongoing physiological functions, with levels typically ranging from 193-824 ng/dL in males aged 18-99.[2] It sustains muscle mass and strength by enhancing protein synthesis in skeletal muscle, promoting a lean body composition with reduced fat accumulation, particularly in the abdominal area.[1] Bone health is preserved through increased mineral density, helping to prevent osteoporosis, while erythropoiesis is stimulated, leading to higher red blood cell counts and hematocrit levels compared to females.[30] Reproductive functions continue via support for spermatogenesis and libido, ensuring fertility and sexual function.[1] With advancing age, testosterone levels gradually decline by about 1-2% per year after the third decade, contributing to reduced muscle mass (sarcopenia), lower bone density, increased fat distribution, diminished libido, and smaller testicular size.[1] In females, adult testosterone supports modest libido and muscle maintenance, but levels remain low at less than 40 ng/dL.[2] This age-related decline underscores testosterone's role in long-term physiological homeostasis, influencing overall vitality and well-being.[30]Reproductive and Secondary Sexual Characteristics
Testosterone plays a pivotal role in the development and maintenance of the male reproductive system, beginning in utero where it promotes the differentiation of Wolffian ducts into structures such as the epididymis, vas deferens, and seminal vesicles, while also facilitating testicular descent in the final months of gestation.[1] In adult males, it is essential for spermatogenesis, acting primarily through Leydig cells to produce high intratesticular concentrations—approximately 40 times higher than serum levels—that support Sertoli cell function, the blood-testis barrier, meiosis, and spermiation.[31] Deficiency in testosterone leads to impaired sperm production, reduced motility, and altered sperm proteome, as evidenced by proteomic studies showing differential expression of proteins like AKAP3 involved in capacitation.[31] Conversely, maintaining physiological levels ensures fertility, with exogenous administration replicating these effects at adult male concentrations.[32] During puberty, testosterone drives the maturation of male reproductive organs, including enlargement of the penis, testes, and prostate, alongside increased libido through stimulation by luteinizing hormone (LH).[2] It also regulates prostate growth and seminal fluid production, contributing to overall reproductive function.[1] In females, testosterone is produced by the ovaries and adrenal glands, supporting ovarian follicle development and converting to estradiol for estrogenic effects, though excess levels, as in polycystic ovary syndrome (PCOS), disrupt ovulation and cause infertility.[2] Secondary sexual characteristics in males emerge prominently during puberty under testosterone's influence, including the growth of pubic, axillary, and facial hair via conversion to dihydrotestosterone (DHT) by 5-alpha-reductase, deepening of the voice due to laryngeal cartilage enlargement, and increased muscle mass and bone density for skeletal maturation.[1] These changes enhance physical prowess and sexual dimorphism, with testosterone also boosting red blood cell production to support oxygen delivery.[2] In females, normal low levels contribute to libido without pronounced secondary traits, but hyperandrogenism results in hirsutism, acne, and clitoral enlargement, altering typical female characteristics.[1] Across both sexes, testosterone's actions on these traits are mediated by androgen receptors, underscoring its role in sexual differentiation and adult phenotype maintenance.[1]Health and Medical Aspects
Immune System and Inflammation
Testosterone exerts immunomodulatory effects on the immune system, primarily acting as an immunosuppressant that influences both innate and adaptive immunity.[33] This role contributes to observed sex differences in immune responses, with higher testosterone levels in males associated with reduced antibody production and dampened inflammatory reactions compared to females.[34] Low testosterone levels, conversely, correlate with heightened systemic inflammation, as evidenced by elevated markers such as C-reactive protein (CRP) and pro-inflammatory cytokines in hypogonadal men.[35] A key aspect of testosterone's influence is its anti-inflammatory activity, which suppresses the production of pro-inflammatory cytokines including tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1 beta (IL-1β).[35] For instance, testosterone therapy in androgen-deficient men has been shown to significantly reduce circulating levels of CRP, IL-1β, TNF-α, and leptin after six months of treatment.[35] In vitro studies further demonstrate that physiological concentrations of testosterone (around 10 ng/mL) inhibit TNF-α and interferon-gamma (IFN-γ) secretion by human macrophages stimulated with immune complexes, without affecting anti-inflammatory IL-10 or other chemokines like CXCL10.[36] These effects occur partly through downregulation of Fcγ receptor expression on macrophages, limiting hyperinflammatory responses.[36] Mechanistically, testosterone modulates immune cell function via androgen receptors, promoting regulatory cells such as myeloid-derived suppressor cells (MDSCs) while suppressing T-cell and B-cell activation.[33] In pathogen-exposed populations, higher endogenous testosterone is linked to downregulated cytokine responses following T-cell stimulation (e.g., reduced GM-CSF and IL-8), suggesting a trade-off that prioritizes energy allocation away from costly immune activation.[34] Additionally, testosterone inhibits adipose tissue-derived inflammation by reducing macrophage infiltration and monocyte chemoattractant protein-1 (MCP-1) expression, thereby mitigating obesity-related chronic inflammation.[35] Clinically, these properties may explain increased male susceptibility to certain infections. Low testosterone levels have been associated with more severe outcomes in infections such as COVID-19, potentially due to impaired regulation of inflammatory responses.[37][38] However, in contexts of excessive inflammation, such as cytokine storms, testosterone replacement in hypogonadal patients could attenuate disease burden by curbing pro-inflammatory pathways.[33] Conversely, androgen deprivation therapy enhances immune responses and has been associated with lower infection risks in some studies.[33] Overall, testosterone's balancing act on inflammation underscores its protective role against chronic conditions like metabolic syndrome and cardiovascular disease when maintained at optimal levels.[35]Therapeutic Uses
Testosterone is primarily used therapeutically to treat hypogonadism in males, a condition characterized by deficient testosterone production due to testicular failure (primary hypogonadism) or hypothalamic-pituitary dysfunction (hypogonadotropic hypogonadism).[39] Replacement therapy restores physiological levels, alleviating symptoms such as reduced libido, erectile dysfunction, fatigue, decreased muscle mass, increased body fat, and low bone density.[40] Clinical studies demonstrate improvements in sexual function, including enhanced erectile function and frequency of sexual activity, as well as mood and energy levels in hypogonadal men.[41] Bone mineral density increases with therapy, reducing osteoporosis risk, while lean body mass rises by approximately 2 kg and fat mass decreases.[41] In adolescent males, testosterone therapy addresses delayed puberty by promoting the development of secondary sexual characteristics, such as genital growth and pubic hair, when administered under medical supervision in combination with formulations like testosterone enanthate.[39] Therapy is typically short-term to initiate puberty, with careful monitoring to avoid premature epiphyseal closure.[42] Hormone therapy for biological females with gender dysphoria employs testosterone to induce male secondary sex characteristics while suppressing female ones.[43] Desired effects include deepening of the voice, increased facial and body hair, clitoral enlargement, fat redistribution to a more android pattern, cessation of menses, and enhanced muscle mass and strength.[43] However, testosterone is not a reliable contraceptive, as ovulation may persist despite amenorrhea in up to one-third of individuals.[44] Additional contraceptive measures are recommended if pregnancy is to be avoided.[45] Dosing aims to achieve male physiological ranges (300–1000 ng/dL), with options for gradual titration to align with individual goals and minimize risks like acne, erythrocytosis, vaginal atrophy, sleep apnea, lipid profile changes, and pattern hair loss.[46][47] This therapy significantly reduces gender dysphoria, depression, and suicidality, as evidenced by randomized trials.[48] Historically, testosterone was used palliatively for advanced inoperable breast cancer in women, where it was thought to inhibit tumor growth through androgenic effects, but this is no longer standard practice.[39][49] Off-label uses include managing hypoactive sexual desire disorder in postmenopausal women or those post-oophorectomy, often combined with estrogen to improve libido, sexual satisfaction, and orgasm frequency.[42] It may also alleviate vasomotor symptoms of menopause, such as hot flashes, though evidence for this is limited.[42] In both sexes, therapy enhances erythropoiesis, increasing hemoglobin by 10–20% in deficient individuals.[41] Recent meta-analyses (as of 2025) indicate that testosterone replacement therapy does not increase the risk of prostate cancer incidence or progression in appropriately selected patients.[50]Associated Disorders
Disorders associated with testosterone imbalance encompass conditions arising from either deficiency (hypogonadism) or excess (hyperandrogenism), affecting both sexes and often linked to broader metabolic, reproductive, and cardiovascular health issues. In men, testosterone deficiency syndrome, or hypogonadism, manifests as reduced serum testosterone levels below 300 ng/dL, leading to symptoms including erectile dysfunction, diminished libido, fatigue, depression, decreased bone density, and loss of muscle mass.[51] This condition is prevalent in aging populations, with late-onset hypogonadism affecting up to 2.1% of men over 40, and is strongly correlated with comorbidities such as obesity, type 2 diabetes, metabolic syndrome, and cardiovascular disease, where low testosterone exacerbates insulin resistance and dyslipidemia.[41] For instance, epidemiological data indicate that men with testosterone levels in the lowest quartile have a 1.5- to 2-fold increased risk of all-cause mortality, independent of age and other risk factors.[52] Testosterone replacement therapy, while beneficial for alleviating hypogonadal symptoms, carries risks of associated disorders, including erythrocytosis (hematocrit >54%), which occurs in 10-40% of treated patients and heightens thrombotic event risks; prostate gland enlargement or worsening of benign prostatic hyperplasia; and potential stimulation of occult prostate cancer, though recent evidence (as of 2025) shows no direct causation or increased risk.[53][50] Additionally, low testosterone is implicated in osteoporosis and frailty in older men, with studies showing a 20-30% higher fracture risk in hypogonadal individuals due to impaired bone mineralization.[51] Genetic conditions like Klinefelter syndrome (47,XXY karyotype) inherently cause primary hypogonadism through testicular failure, resulting in infertility and elevated gonadotropin levels alongside low testosterone.[54] In women, hyperandrogenism—elevated testosterone and other androgens—primarily stems from polycystic ovary syndrome (PCOS), affecting 5-10% of reproductive-age women and characterized by hirsutism, acne, alopecia, menstrual irregularities, and infertility due to ovarian androgen overproduction.[55] PCOS is associated with insulin resistance, leading to a 3- to 7-fold increased risk of type 2 diabetes and metabolic syndrome, with hyperandrogenemia directly contributing to these outcomes via enhanced visceral fat accumulation and lipotoxicity.[55] Other causes include congenital adrenal hyperplasia, where enzyme deficiencies (e.g., 21-hydroxylase) cause excessive adrenal androgen synthesis, resulting in virilization, ambiguous genitalia in females, and salt-wasting crises. Ovarian hyperthecosis, a rarer disorder, involves stromal hyperplasia leading to markedly elevated testosterone (>200 ng/dL), often postmenopausally, and is linked to severe hirsutism and insulin resistance. Androgen excess in women also correlates with cardiovascular risks, including hypertension and dyslipidemia, mirroring patterns seen in male hypogonadism but driven by supraphysiological levels.[55] Both hypo- and hyperandrogenism contribute to mood disorders; low testosterone in men is tied to depressive symptoms via altered neurotransmitter function, while in women, the relationship follows a parabolic curve where both low and high levels correlate with increased depression risk.[56] Therapeutic interventions must balance these risks, as exogenous testosterone in women for hypoactive sexual desire disorder can induce hyperandrogenic side effects like voice deepening or clitoromegaly if not monitored.[57] Overall, these disorders underscore testosterone's role in endocrine homeostasis, with management focusing on underlying etiologies to mitigate secondary complications.[58]Behavioral and Social Effects
Sexual Arousal and Relationships
Testosterone plays a central role in regulating sexual arousal and desire in both men and women, primarily through its actions on the central nervous system and peripheral tissues. In men, low testosterone levels are strongly associated with reduced sexual desire, which is the most prominent symptom of hypogonadism, while testosterone replacement therapy (TRT) has been shown to improve libido in a dose-dependent manner.[59] For instance, transdermal testosterone administration in hypogonadal men significantly increased sexual desire scores compared to placebo, with effects observable within 30 days.[59] In terms of erectile function, testosterone enhances penile blood flow and nitric oxide-mediated vasodilation, particularly in older men with low baseline levels, though its impact is modest and often overshadowed by age-related factors or comorbidities.[60] Meta-analyses indicate that TRT yields an approximately 8% improvement in erectile function scores on the International Index of Erectile Function (IIEF), compared to 20-50% efficacy from phosphodiesterase-5 inhibitors alone.[60] In women, testosterone contributes to sexual arousal by influencing genital responsiveness and subjective lust, with effects mediated through androgen receptors in the brain and vaginal tissues. A controlled study in premenopausal women demonstrated that sublingual testosterone administration led to a significant increase in vaginal pulse amplitude—a measure of genital arousal—3 to 4.5 hours after peak plasma levels, accompanied by heightened subjective sensations of genital arousal and sexual lust.[61] For postmenopausal women with hypoactive sexual desire disorder, testosterone therapy modestly boosts desire and satisfaction, though long-term safety data remain limited and effects are smaller than in men.[62] Mechanisms involve aromatization to estrogen in the brain, which supports libido, and direct androgen effects on clitoral and vaginal smooth muscle function.[62] Beyond individual arousal, testosterone levels influence relationship dynamics and pair bonding, particularly in men, where committed romantic partnerships are associated with reduced circulating testosterone. Studies consistently show that men in long-term relationships have approximately 21% lower testosterone levels than single men, independent of marital status or age.[63] This pattern aligns with the challenge hypothesis, suggesting a shift from mating effort (higher testosterone) to parenting and bonding efforts (lower testosterone) in paired individuals.[64] In early-stage relationships, testosterone levels may remain elevated similar to singles, but they decline with relationship duration and commitment.[65] Fathers exhibit even lower levels, up to 42% below singles, further supporting the role of testosterone in modulating social and reproductive behaviors.[63] These associations highlight testosterone's broader impact on maintaining monogamous bonds, though causality and mechanisms—potentially involving oxytocin interactions—require further elucidation.[66]Aggression and Motivation
Testosterone has been extensively studied for its association with aggressive behavior, though the relationship is nuanced and generally weak. Meta-analytic evidence indicates a small positive correlation between baseline testosterone levels and aggression in humans, with an effect size of r = 0.054 across diverse populations and measures of aggression, such as self-reports, laboratory tasks, and real-world incidents.[67] This association strengthens slightly with acute fluctuations in testosterone, yielding r = 0.108, suggesting that dynamic changes—such as those during competition or stress—may play a more prominent role than steady-state levels.[67] Earlier meta-analyses similarly reported weak positive links, emphasizing that testosterone accounts for only a modest portion of variance in aggressive outcomes, often moderated by contextual factors like social status or provocation. In specific contexts, such as competitive or dominance-related scenarios, testosterone appears to facilitate aggressive responses as part of a broader behavioral repertoire. For instance, elevations in testosterone during interpersonal challenges correlate with increased willingness to engage in confrontational behaviors, potentially serving an adaptive function in resource competition.[68] However, this link is not causal in a straightforward manner; experimental manipulations, like testosterone administration, show inconsistent effects on aggression, with some studies finding heightened irritability or risk-taking but no universal increase in physical aggression.[67] Factors such as baseline hormone levels, genetic predispositions, and environmental cues (e.g., winning or losing a contest) further modulate these effects, highlighting testosterone's role as one influence among many in the complex etiology of aggression.[69] Regarding motivation, testosterone is implicated in driving goal-directed behaviors, particularly those oriented toward status enhancement and dominance. Pharmacological studies demonstrate that exogenous testosterone administration boosts status-seeking motivation in men, increasing persistence in tasks where social rank can be improved, such as economic games or competitive efforts.[70] This aligns with neurobiological evidence linking testosterone to reward processing via interactions with dopaminergic pathways, which underpin motivation and reinforcement learning.[71] In power-motivated individuals, rising testosterone levels facilitate dominance pursuits, while declines after setbacks (e.g., defeat) may reduce motivational drive without directly causing withdrawal.[72] Testosterone's motivational effects extend to persistence against adversity, where it promotes continued effort in the face of stronger opponents, potentially by altering risk assessment and reward valuation in the brain.[73] However, these influences are context-dependent; for example, testosterone can reduce prosocial motivations in favor of self-interested actions, as seen in reduced generosity toward distant others in decision-making paradigms.[74] Overall, testosterone contributes to a dominance-oriented motivational framework, integrating aggression with approach behaviors to navigate social hierarchies, though individual differences in hormone sensitivity and environmental demands shape its expression.[71]Paternal and Social Roles
Testosterone plays a significant role in modulating paternal behavior in humans, often exhibiting a trade-off between mating competition and caregiving. Longitudinal studies have shown that expectant fathers exhibit lower testosterone levels even during the prenatal period compared to nonfathers.[75] Following the transition to parenthood, men who become fathers experience substantial declines in testosterone levels. For instance, newly partnered fathers exhibit a median decrease of 26% in waking testosterone and 34% in evening testosterone, compared to smaller declines of 12% and 14% in single nonfathers. These reductions are more pronounced in fathers who spend three or more hours daily on childcare, suggesting that active paternal involvement suppresses testosterone to facilitate nurturing behaviors.[76] Furthermore, baseline testosterone levels predict the likelihood of entering fatherhood, with higher initial levels in single nonfathers associated with greater odds of becoming partnered fathers (odds ratio = 1.21). Once fatherhood is established, lower testosterone correlates with increased paternal caregiving and desired involvement (r = -0.27 and r = -0.26, respectively). This pattern extends to neurobiological responses, where smaller testes volume—a proxy for lower testosterone production—is inversely related to nurturing-related brain activity in the ventral tegmental area (VTA) when fathers view images of their own child (r = -0.48). Such VTA activation, part of the brain's reward system, positively predicts caregiving behaviors (r = 0.28). In fathers, stable but elevated testosterone levels are linked to reduced parent-infant synchrony and affectionate touch, indicating an inhibitory effect on bonding, whereas in mothers, higher testosterone can enhance the positive influence of oxytocin on tactile interactions.[77][78] In social contexts, testosterone influences behaviors aimed at enhancing or maintaining status within hierarchies, often promoting both prosocial and antisocial actions depending on the situation. Exogenous testosterone administration in men increases punishment of unfair resource divisions in economic games, reflecting antisocial status defense, while simultaneously boosting rewards for generous offers, demonstrating prosocial status enhancement. This dual effect supports the social status hypothesis, where testosterone drives context-dependent behaviors to elevate perceived rank. For example, in competitive interactions, winning elevates preferences for high-status goods, though basal testosterone changes may not always directly predict such shifts.[79] Testosterone's impact on social roles also varies by an individual's position in the hierarchy. Among men in established groups, such as sports teams, higher testosterone promotes submissiveness (e.g., accepting smaller offers in negotiations) among lower-status juniors to facilitate alliance-building and upward mobility, but fosters dominance (e.g., demanding larger shares) among high-status seniors to reinforce position. This strategic modulation aligns with evolutionary pressures for status-seeking, where testosterone amplifies behaviors that optimize social advancement without unnecessary conflict. Overall, pair-bonded and paternal men tend to have lower testosterone than single or childless counterparts, underscoring its role in shifting priorities from competition to cooperative social and familial roles.[80]Measurement and Distribution
Measurement Techniques
Testosterone levels in the body are typically measured in serum or plasma to assess total, free, or bioavailable concentrations, with techniques varying in accuracy and clinical applicability. The most widely used methods for total testosterone include immunoassays and mass spectrometry-based approaches. Immunoassays, such as enzyme-linked immunosorbent assay (ELISA) or chemiluminescent immunoassays, are routine in clinical laboratories due to their simplicity and speed but often overestimate or underestimate levels, particularly at low concentrations, due to cross-reactivity with other steroids.[81] In contrast, liquid chromatography tandem mass spectrometry (LC-MS/MS) is considered the gold standard for total testosterone measurement, offering high specificity and sensitivity by separating testosterone from interfering compounds before quantification.[81][82] The Centers for Disease Control and Prevention (CDC) certifies laboratories using LC-MS/MS assays that meet standardization criteria, ensuring measurement variability within ±6.4% coefficient of variation (CV).[82] For optimal accuracy, total testosterone should be measured in the early morning (between 7-10 a.m.) under fasting conditions, as levels exhibit diurnal variation with peaks in the morning.[82] Diagnosis of testosterone deficiency typically requires two separate measurements on different occasions to account for intra-individual variability.[82] In specialized settings, gas chromatography-mass spectrometry (GC-MS) serves as an alternative reference method, though LC-MS/MS is preferred for its efficiency and lower sample volume requirements.[81] Free testosterone, the unbound fraction biologically active in circulation, constitutes about 1-2% of total testosterone and is measured when binding proteins like sex hormone-binding globulin (SHBG) are altered, such as in obesity or liver disease. Direct measurement via equilibrium dialysis is the reference method, involving separation of free hormone across a semi-permeable membrane while maintaining physiological equilibrium, followed by LC-MS/MS quantification of the dialysate.[81][83] This technique avoids assumptions about binding affinities but is labor-intensive and costly. Alternatively, free testosterone can be calculated using total testosterone, SHBG, and albumin levels via equations like the Vermeulen formula, providing a practical approximation with good correlation to dialysis results in most cases.[81] Bioavailable testosterone, encompassing free plus weakly bound fractions, is assessed by precipitating SHBG and measuring the supernatant with LC-MS/MS, useful for evaluating androgen status in SHBG excess.[81] Salivary assays have been explored for non-invasive free testosterone estimation but lack standardization and are not recommended for clinical diagnosis.[81]Population Distribution
Testosterone levels exhibit significant variation across human populations, primarily influenced by sex, age, ethnicity, and geography. In adult males, circulating total testosterone concentrations typically range from 264 to 916 ng/dL (9.2 to 31.8 nmol/L) in healthy, non-obese individuals aged 19 to 39 years, based on harmonized reference ranges derived from multiple cohort studies.[84] In contrast, adult females maintain much lower levels, with the 10th to 90th percentiles spanning 7.1 to 49.8 ng/dL (0.25 to 1.73 nmol/L), reflecting the hormone's role in ovarian function and minimal adrenal production compared to males.[13] These sex-based differences arise from gonadal production, where testes in males synthesize over 95% of circulating testosterone, while in females, the ovaries and adrenals each contribute approximately 25% directly, with the remainder from peripheral conversion.[2][85] Age profoundly impacts testosterone distribution, particularly in males, where levels peak during late adolescence and early adulthood before declining gradually. In males aged 19 to 39 years, middle tertile ranges are approximately 409 to 558 ng/dL for ages 20-24, 413 to 575 ng/dL for 25-29, and 359 to 498 ng/dL for 30-34, with a continued decline of about 1-2% per year thereafter.[86] By age 70 and older, average levels often fall below 300 ng/dL, contributing to age-related hypogonadism in up to 30% of older men.[82] In females, levels remain relatively stable post-puberty but decrease during menopause, dropping to 5-20 ng/dL, influenced by ovarian follicle depletion.[13] Population surveys indicate that these age trajectories are consistent across large cohorts, though individual variability is high due to factors like body mass index and lifestyle.[87] Ethnic and racial differences in testosterone distribution have been observed in multiple studies, often after adjusting for age and body composition. Non-Hispanic Black males tend to have higher total and free testosterone levels than non-Hispanic White males, with differences of approximately 0.39 ng/mL (13%) in the 20-69 age range and free testosterone elevated by 4.07 pg/mL.[88][89] Mexican-American males exhibit the highest concentrations among major U.S. ethnic groups, surpassing both Black and White populations, while Asian males in the U.S. show levels similar to Whites.[90] These variations may stem from genetic factors, such as differences in sex hormone-binding globulin, though environmental influences like diet cannot be ruled out.[91] In children and adolescents, ethnic disparities are more pronounced, with Black youth displaying earlier and higher testosterone surges during puberty.[92] Geographical and secular trends further modulate population distribution. Asian males in Hong Kong and Japan have total testosterone levels about 20% higher than U.S. counterparts, potentially linked to regional differences in body fat or endocrine disruptors.[93] Over time, U.S. and European studies report a population-level decline in male testosterone of 1% per year since the 1980s, independent of aging, attributed to rising obesity, sedentary lifestyles, and environmental exposures.[94] Such trends underscore the interplay of biological and modifiable factors in shaping testosterone epidemiology across global populations.| Age Group (Males) | Middle Tertile Total Testosterone (ng/dL) | Source |
|---|---|---|
| 20-24 years | 409-558 | [86] |
| 25-29 years | 413-575 | [86] |
| 30-34 years | 359-498 | [86] |
| 70+ years | <300 (average) | [82] |
| Ethnic Group (Adult Males, Age-Adjusted) | Average Difference vs. White Males | Source |
|---|---|---|
| Non-Hispanic Black | +0.39 ng/mL total T | [88] |
| Mexican-American | Highest overall concentrations | [90] |
| Asian (U.S.) | Similar to White | [90] |