Metabolic Messengers: Testosterone

Abstract

Testosterone, discovered during the endocrine gold rush of the 1930s, was the first hormone chemically synthesized for replacement therapy. In both males and females, testosterone functions directly via the androgen receptor (AR) and indirectly as a prohormone, converted by aromatase into 17β-estradiol (estradiol), which activates estrogen receptors ERα or ERβ. Testosterone is also metabolized to dihydrotestosterone (DHT)—a potent, non-aromatizable AR agonist—via steroid 5α-reductases. Testosterone and its metabolites signal through AR- and ER-mediated genomic and rapid, nongenomic actions. Long recognized for its role as a sex hormone, mounting evidence underscores the role of testosterone in the regulation of system metabolism in both males and females. Here, we highlight key milestones in the history of testosterone discovery and therapy, and synthesize current understanding of testosterone as a key messenger promoting metabolic homeostasis in preclinical models and humans.

In 1935, Ernst Laqueur isolated testosterone from testicular extracts. Soon after, Adolf Butenandt and Leopold Ruzicka synthesized testosterone from cholesterol, leading to the commercialization of testosterone propionate in 1937, the first chemically synthesized hormone marketed for hormone replacement therapy.¹

Before radioimmunoassays enabled accurate testosterone measurement, hypogonadism diagnosis relied on clinical signs and symptoms. Consequently, most diagnosed men had severe testosterone deficiency, presenting with absent or underdeveloped secondary sexual characteristics, gynecomastia, and small testes. In these patients, testosterone therapy dramatically improved physical appearance, secondary sex characteristics, and sexual function, leaving minimal doubt regarding its therapeutic benefits.

The introduction of convenient testosterone transdermal gels in 2002, coupled with aggressive direct-to-consumer marketing, and the proliferation of men’s health clinics across the United States, triggered a dramatic surge in testosterone prescriptions, and raised concerns among regulatory agencies regarding widespread off-label testosterone use.²

Following the Institute of Medicine’s conclusion that evidence supporting testosterone’s benefits and safety in older hypogonadal men was insufficient, the U.S. National Institute on Aging sponsored the Testosterone Trials (TTrials), a series of seven linked studies evaluating testosterone therapy’s impact on sexual and physical function, vitality, bone strength, and anemia.³

Prompted by conflicting reports of cardiovascular events in testosterone-treated men,^4,5 the U.S. Food and Drug Administration (FDA) conducted an extensive review of testosterone’s cardiovascular safety in 2015, and directed testosterone manufacturers in the U.S. to place a black box warning on testosterone products and to conduct a randomized controlled trial (RCT) to assess its cardiovascular risks.

The subsequent TRAVERSE trial confirmed testosterone’s long-term cardiovascular safety and clarified its beneficial effects on sexual function, mood, and anemia in older hypogonadal men.⁶ Additionally, the Testosterone for Type 2 Diabetes Mellitus (T4DM) trial demonstrated that combining testosterone therapy with lifestyle intervention produced anti-diabetic effect in obese men compared to lifestyle alone.

The findings from TTrials, TRAVERSE, T4DM, and other recent studies have provided high quality evidence of the metabolic benefits and low risk of serious adverse events associated with testosterone therapy. Reflecting this new evidence, the FDA revised its black box warning regarding testosterone and cardiovascular risks in 2025.

Concurrently, the discoveries and cloning of androgen receptors (AR), estrogen receptors (ERα and ERβ), CYP19A1 aromatase, steroid 5α-reductases, and the identification of testosterone’s rapid extranuclear signaling mechanisms have significantly expanded our understanding of testosterone’s diverse actions and its critical role as a metabolic messenger.

Figure 1 summarizes key milestones in the history of testosterone discovery and therapeutic application.

Figure 1: Testosterone: From discovery to metabolic messenger.

Timeline of milestones in testosterone’s discovery and therapeutic developments (upper panel), to the understanding of its role as a metabolic messenger (lower panel).

Mechanisms of testosterone action

In males, testosterone is mainly secreted by the Leydig cells of the testes under control of pituitary luteinizing hormone (LH). In females of reproductive age, testosterone is produced by the ovaries (25%), the adrenal glands (25%) and in peripheral tissues (50%) by conversion of circulating androstenedione.⁷ With advancing age, testosterone production and levels decline in men and women.

Testosterone functions as a hormone, binding the androgen receptor (AR), to trigger the development of the male internal reproductive organs and secondary sex characteristics, regulate metabolism and promote skeletal muscle and bone growth, hematopoiesis, and spermatogenesis. Testosterone also acts as a prohormone, undergoing peripheral conversion to 17β-estradiol (estradiol) by aromatase, that binds to estrogen receptors ERα or ERβ and is an important messenger of testosterone’s action in metabolic homeostasis in both males and females as discussed below. In men, approximately 80% of circulating estradiol originates from the aromatization of testosterone in peripheral tissues, while direct testicular production accounts for the remaining 20%.⁷ Testosterone is also converted to dihydrotestosterone (DHT)—a non-aromatizable androgen—by a family of steroid 5α reductases; DHT binds AR to promote the development of the male external genitalia, and prostate.

Plasma membrane ERs and ARs are the first point of contact of testosterone and its metabolites with the cell signaling machinery. Rapid sex steroid signaling is a prerequisite for downstream AR and ERs nuclear translocation and transcriptional activity (reviewed in^8,9). Membrane-localized AR and ERs monomers activate kinase cascades—such as MAPK and PI3K/Akt pathways—that in turn potentiate the actions of their dimeric nuclear counterparts, usually via phosphorylation. This integrated model of AR and ERs signaling allows steroid-dependent early membrane effects to enhance steroid-dependent late nuclear actions involved in metabolic homeostasis. Membrane-localized and nuclear AR and ERs can also collaborate by activating different and synergistic pathways that target the same molecular endpoint.¹⁰

Membrane-initiated rapid AR and ERs action can also be independent of nuclear actions, producing non-genomic signaling events that modulate nitric oxide production, G-protein-coupled receptor activity, ion channel activity and calcium flux.^8,11 Some rapid actions of testosterone can also be mediated via direct binding to L-type Ca2+ channels, independent of AR and ERs.¹² Additionally, mitochondrial AR and ERs regulate mitochondrial activity and dynamics.

Figure 2 summarizes the molecular mechanisms of testosterone action in metabolism.

Figure 2. Schematic representation of testosterone signaling via AR and ERs in metabolism.

Testosterone functions directly via AR and after conversion by aromatase to estradiol (E₂), which activates ERs and dihydrotestosterone (DHT) via steroid 5α-reductases that activates AR. Upon ligand stimulation a subpopulation of AR and ERs localize to membrane (10%) via palmitoylation and the remaining AR and ERα translocate to the nucleus. Membrane AR and ERs are organized into functional lipid raft signalosomes to initiate rapid signal transduction from the cell surface, directly binding membrane-associated small G-proteins, nonreceptor tyrosine kinases (SRC), PI3K, AMPK and ion channels to activate downstream signaling pathways. Nuclear AR and ERs are important substrates of the same signaling pathways they rapidly activate and are phosphorylated in response to activation of mitogenic signaling pathways (i.e. MAPK, AKT), which modulates their transcriptional activity. Steroid action via mitochondrial AR and ERs regulate mitochondrial activity and dynamics.

Tissue-specific metabolic effects of testosterone in the male.

Here, we synthesize current understanding of testosterone as an important messenger that regulates metabolic homeostasis. Figure 3 summarizes the tissue-specific metabolic effects of testosterone in humans and mice.

Figure 3. Summary of testosterone’s effects as a metabolic messenger in men and male mice.

In men and male mice , testosterone promotes metabolic homeostasis as a hormone, binding the AR, and following conversion to dihydrotestosterone (DHT). Testosterone is also a prohormone undergoing conversion to 17β-estradiol (E₂) by aromatase, that binds to estrogen receptors ERα or ERβ and is a critical messenger of testosterone’s action in metabolic homeostasis.

Skeletal muscle.

Testosterone dose-dependently increases skeletal muscle mass, strength, and power in hypogonadal¹³ and eugonadal men.¹⁴ Administration of supraphysiologic testosterone doses further enhances muscle size and strength in eugonadal men even without exercise; these anabolic effects are augmented by resistance training.¹⁵ In men, testosterone induces hypertrophy of both oxidative (mitochondria-rich) and glycolytic (mitochondria-poor, type II) muscle fibers, and increases myonuclei and satellite cells.¹⁶ Testosterone also increases muscle mass by enhancing postprandial protein synthesis in men.¹⁷ Although both testosterone and DHT promote muscle mass and strength, conversion of testosterone to DHT is not required for its anabolic effects in men.¹⁸

Testosterone improves insulin sensitivity in men with and without type 2 diabetes (T2D).¹⁹ These effects are dependent on both ERα and AR pathways. Testosterone increases peroxisome proliferator-activated receptor-δ coactivator 1-α (PGC1α) expression, which stimulates mitochondrial biogenesis and oxidative muscle fibers. Men with low testosterone have reduced skeletal muscle PGC1α expression,²⁰ associated with insulin resistance.²¹ Estradiol could mediate this effect, as estradiol treatment increases muscle PGC1α in men.²²

Further, estradiol enhances muscle insulin sensitivity via ERα in mice by improving mitochondrial quality and function.^23,24 Conversion of testosterone to DHT is also required for testosterone insulin sensitization of liver and skeletal muscle: dual inhibition of 5α-reductase types 1 and 2—but not type 2 alone—induces hepatic lipid accumulation and insulin resistance in men.^25,26 In male mice, testosterone also enhances skeletal muscle insulin sensitivity via AR by promoting glucose uptake and utilization, glycolysis, and glycogen synthesis.^27–29 Muscle-specific overexpression of AR induces hypertrophy of glycolytic muscle fibers, increasing glucose metabolism.³⁰ Together these findings show that testosterone increases both oxidative and glycolytic fibers in mice via ERα and AR pathways.

In male mice, testosterone increases muscle progenitor cell numbers by driving mesenchymal progenitor differentiation toward the myogenic lineage while suppressing their differentiation into adipogenic lineage.³¹ Liganded AR complexes with β-catenin, translocate into the nucleus, and upregulate Wnt-target genes, including follistatin, which inhibits TGFβ signaling and stimulates myogenesis.^32–34 Testosterone also promotes male mouse myoblast proliferation by increasing polyamine synthesis.^35,36 Furthermore, testosterone prevents muscle atrophy by suppressing the C/EBPδ/myostatin pathway.³⁷

Adipose tissue.

In men, testosterone deficiency promotes whole-body and abdominal adipose tissue accumulation and contributes to the development of metabolic syndrome.³⁸ Conversely, chronic—but not acute—testosterone treatment increases energy expenditure and lipid oxidation.^39,40 Therefore the ability of testosterone to decrease whole-body and abdominal adipose tissue likely involves a delayed metabolic effect, via increased muscle mass.

Similarly, orchiectomy induces adipogenesis and adipocyte hypertrophy in male mice, leading to fat mass expansion and increased susceptibility to diet-induced obesity but testosterone administration reverses these effects.⁴¹

Testosterone inhibits whole-body and visceral adipose tissue accumulation through estradiol-mediated activation of ERα in male mice and men. Orchiectomized male rodents treated with estradiol remain lean, whereas those treated with the non-aromatizable androgen DHT become obese.⁴² In addition, the ability of testosterone to suppress lipogenic genes is maintained in adipose and liver of TFM AR deficient mouse because ERα is still active.^29,43 Testosterone anti-adipogenic effect in men is blunted by co-administration of anastrozole, an aromatase inhibitor, which inhibits conversion to estradiol and action via ERα.⁴⁴ Inactivating mutations in the aromatase gene are associated with increased abdominal adiposity in men.^45,46 Mechanistically, estradiol generated from testosterone suppresses adiposity via ERα signaling in multiple tissues. In mice, ERα activation in mesenchymal stem cells inhibits adipogenic differentiation⁴⁷ while ERα action in adipocytes represses lipoprotein lipase expression^48,49 together reducing lipid storage.

Testosterone’s anti-adiposity effects also require AR activation. Male mice with genetic AR disruption develop late-onset visceral obesity and insulin resistance.^50,51 Testosterone suppression visceral fat is mediated in part through indirect AR signaling in skeletal muscle: muscle-specific AR overexpression in male rats increases muscle mass, elevates metabolic rate, and reduces adiposity.³⁰ In contrast, adipocyte-specific AR deletion in male mice does not alter fat mass, suggesting that direct AR signaling in adipocytes is not required.⁵²

Pancreatic β-cells.

Testosterone also modulates β-cell function. Human and rodent male β cells express 5α-reductase-1 and aromatase.⁵³ After conversion to DHT and estradiol, testosterone enhances glucose-stimulated insulin secretion (GSIS). Male mice lacking β-cell AR (βARKO) develop β-cell dysfunction, fail to compensate for insulin resistance, and become hyperglycemic.⁵⁴ In βARKO islets, genes regulating insulin secretion and inflammation are dysregulated.⁵⁵ Thus, in the absence of AR in β cells, testosterone cannot maintain normoglycemia, demonstrating the importance of β cell AR signaling to glucose homeostasis in male mice.

In male mice and human β cells, DHT activation of AR in the vicinity of the plasma membrane amplifies GLP-1–mediated insulin secretion.^11,54,56 DHT and GLP-1 collaborate to enhance Gα_s recruitment to the G protein-coupled GLP-1 receptor and AR complexes to activate transmembrame adenylate cyclase and increase cAMP production. DHT also programs glucose metabolism toward mitochondrial CO₂ production which is converted to HCO₃⁻ to activate a soluble adenylate cyclase to produce cAMP.¹¹

Aromatization of testosterone to estradiol is also essential for enhancing GSIS⁵³ and protecting β cells against streptozotocin-induced damage in mice.⁵⁷ In rodent models of β-cell failure, estradiol protects islets from oxidative stress, glucolipotoxicity, and endoplasmic reticulum stress.^57–60

Bone.

Testosterone contributes to bone mass, strength, and microarchitecture and may partially mediate its systemic metabolic benefits via actions on bone. Testosterone therapy improves bone density, trabecular microarchitecture and mechanical strength.⁶¹ These effects are mediated through aromatization to estradiol acting on ERα, and direct AR signaling.^62,63 Men who are estradiol resistant or deficient by inactivating mutations in ERα or the aromatase gene exhibit impaired skeletal development and early-onset osteoporosis.^45,64 In men, estradiol primarily prevents bone resorption, while both estradiol and testosterone stimulate bone formation.⁶⁵ Estradiol suppresses osteoclastogenesis by inhibiting RANKL, sclerostin, IL-1β, and TNFα via ERα, while promoting both trabecular and cortical bone formation.⁶¹ Testosterone enhances trabecular bone formation through AR-mediated induction of IGF-1 and TGFβ.⁶¹ In parallel, testosterone-induced increases in muscle mass and strength may secondarily support skeletal loading and bone strength.

Bone is also a metabolically active endocrine organ. In male mice, osteoblast-derived osteocalcin regulates glucose and energy homeostasis.⁶⁶ In healthy men rendered hypogonadal with a GnRH agonist, circulating osteocalcin declines and is restored with testosterone treatment.⁶⁵ Testosterone also increases osteocalcin in men with T2D.⁶⁷ This effect is mediated by testosterone aromatization, as it is recapitulated by estradiol treatment.⁶⁵ Notably, in male mice, osteocalcin regulates testosterone production via signaling on a G protein–coupled receptor expressed in Leydig cells, highlighting bidirectional cross-talk between bone and testicular function.⁶⁸

Blood vessels.

Low testosterone in men is associated with increased carotid artery intima-media thickness, a marker of atherosclerosis,⁶⁹ and with elevated CVD risk.⁷⁰ Similarly, androgen deprivation therapy (ADT) in men with prostate cancer promotes vascular dysfunction⁷¹ and increases CVD risk.⁷² In contrast, higher endogenous testosterone levels are associated with reduced CVD risk.⁷³ Animal models provide causality: Castration accelerates atherosclerosis and testosterone replacement exerts anti-atherogenic effects.¹²

Testosterone is a potent vasodilator in men, and acutely increases coronary flow⁷⁴ by relaxing vascular smooth muscle cells (VSMCs), and stimulating nitric oxide (NO) production in endothelial cells. Thus, testosterone improves flow-mediated dilation (FMD)—an NO-dependent marker of endothelial function.^75,76 These effects are partially mediated by aromatization to estradiol. In healthy men, circulating estradiol, but not testosterone, correlates positively with FMD,⁷⁷ while aromatase inhibition reduces FMD.⁷⁸ Low doses of estrogens in men promote endothelium-dependent vasodilation and lower blood pressure.⁸ In men and mice, these vasodilatory effects occur via nongenomic, membrane-initiated ERα signaling that promotes endothelial NOS phosphorylation and NO production.⁸ The early onset of endothelial dysfunction and coronary artery disease (CAD) in a man with an inactivating ERα mutation further supports the critical role of ERα in testosterone’s vascular actions.^79,80

Testosterone also binds L-type voltage-gated calcium channels in VSMCs, producing vasorelaxation independent of AR or ERs and similar to nifedipine—a drug used to treat hypertension.¹²

Testosterone may also exert anti-atherogenic effects by modulating lipids and inflammation. In men, testosterone levels are negatively associated with total cholesterol, LDL-C, triglycerides, and inflammatory markers including interleukin-6 and C-reactive protein (CRP).⁸¹ Testosterone replacement lowers LDL-C, triglyceride and CRP in hypogonadal men. Testosterone conversion to estradiol contributes to lipid regulation: Men with aromatase mutations display low HDL-C, high LDL-C, and elevated triglycerides—all corrected by estradiol treatment.^45,82 In men, oral estradiol increases HDL-C, and decreases LDL-C and triglyceride.^83,84 These effects occur within a physiological range and conversely, high dose oral estrogens—which decreases testosterone—increase atherothrombotic risk.⁸ In males, sustained estradiol-mediated atheroprotection requires nuclear ERα activity.^85,86 This could be mediated via ERα activation in hepatocytes, which promotes reverse cholesterol transport,⁸⁷ and/or in macrophages, suppressing pro-inflammatory cytokine production and contributing to the resolution of chronic low-grade inflammation.⁸⁸ Testosterone also exert anti-atherogenic effects via AR-dependent pathways by stimulating cholesterol clearance from human macrophages via the master regulator of cellular cholesterol homeostasis liver X receptor-α.⁸⁹ While testosterone reduces HDL-C,⁹⁰ this does not impair cholesterol efflux capacity, a process dependent on AR signaling.⁹¹

Despite the aforementioned positive effects, clinical trials have generally shown a neutral effect of testosterone on CVD outcomes. The Testosterone Effects on Atherosclerosis Progression in Aging Men (TEAAM) trial⁹² and the TTrials,⁹³ found no significant difference in coronary artery calcium progression or carotid artery intima-media thickness between testosterone- and placebo-treated men. Similarly, the TRAVERSE trial reported no difference in major adverse cardiovascular events after 33 months of follow-up.⁶

Heart.

Sex differences in cardiac physiology and failure reflect both chromosomal and hormonal influences.⁹⁴ Under physiological conditions, testosterone generally supports cardiac health. In rats, it enhances myocardial contractility by increasing intracellular calcium via L-type calcium channels,⁹⁵ and promotes cardiomyocyte glucose uptake via CaMKII and AMPK activation.⁹⁶

Many cardioprotective effects of testosterone are estradiol-dependent as suggested by studies in women and female mice. Menopause is associated with increased heart failure risk, myocardial fibrosis, and dysfunction.⁹⁷ In mice, estradiol limits cardiac hypertrophy, fibrosis, apoptosis, and oxidative stress while improving mitochondrial function.⁹⁴ Antihypertrophic effects are mediated by mitochondrial and membrane ERβ⁹⁸, while antiapoptotic effects involve PI3K/Akt activation via ERα.⁹⁹ Estradiol also promotes cardiac mitochondrial biogenesis via G protein-coupled ER signaling.¹⁰⁰

In contrast, in males testosterone acting via AR may promote pathological cardiac remodeling under certain conditions.⁹⁴ Clinical trials suggest a potential beneficial role for testosterone in men with heart failure with improved lean mass, muscle strength, and aerobic capacity, but without noticeable effects on cardiac function.¹⁰¹ Testosterone also shortens the age-related QT interval prolongation in men,¹⁰² reducing arrhythmia risk via rapid, non-genomic AR signaling suppressing L-type calcium and delaying rectifier potassium currents.¹⁰³

Hematopoiesis.

In men, testosterone increases red blood cell production and corrects unexplained anemia¹⁰⁴. Testosterone stimulates erythropoiesis by increasing the number of hematopoietic progenitors in the bone marrow, stimulating erythropoietin, and by suppressing hepcidin transcription via AR signaling in hepatocytes, enhancing iron availability and erythrocyte iron incorporation.¹⁰⁵ Testosterone also increases erythrocyte 2,3-diphosphoglycerate, shifting the oxygen dissociation curve to enhance oxygen delivery to tissues.¹⁰⁶ Together these effects may improve aerobic capacity in heart failure. Testosterone treatment can also produce erythrocytosis; the incidence of erythrocytosis with physiologic testosterone replacement doses was low and not associated with cardiovascular events in randomized trials.¹⁰⁷

In male mice, testosterone expands myeloid progenitor populations and promotes neutrophil release from bone marrow.¹⁰⁸ In men testosterone treatment increases circulating neutrophils and monocytes within the physiological range.¹⁰⁹

Testosterone therapy and prevention of metabolic dysfunction in hypogonadal men

Testosterone levels are inversely correlated with T2D risk. Androgen deprivation therapy (ADT), used in men with prostate cancer, increases the risk of T2D.^72,110 Conversely, in the T4DM trial, combining testosterone therapy with lifestyle intervention in men with impaired glucose tolerance or newly diagnosed T2D was associated with a 50% reduction in the progression from prediabetes to diabetes and a 13% greater remission rate of newly diagnosed T2D compared to lifestyle alone.¹¹¹ The antidiabetic effect was independent of baseline testosterone levels but required achieving serum concentrations ≥500 ng/dL. Similarly, a registry study (using injectable testosterone) showed improved glycemic control, triglyceride, and LDL cholesterol in testosterone-treated hypogonadal men, whereas untreated men exhibited metabolic decline.¹¹² In contrast, in the TRAVERSE¹¹³ and TTrial,¹¹⁴ treatment with transdermal testosterone—which produced lower testosterone levels—showed no significant benefit on the progression from prediabetes to diabetes or on glycemic remission. It is likely that higher testosterone levels and a greater muscle mass increase such as that achieved by injectable testosterone esters may be required to elicit clinically meaningful antidiabetic effect.

A mediation analysis from the T4DM trial concluded that testosterone’s antidiabetic effect is primarily driven by reduction in fat mass.¹¹⁵ However, testosterone-induced increase in skeletal muscle mass—with its associated alterations in muscle fiber types, enhanced insulin-mediated glucose uptake and fatty acid oxidation—as well as testosterone insulinotropic effect on pancreatic β-cell likely also contribute to its anti-diabetic effect.

Role of testosterone in female metabolic homeostasis

In women with polycystic ovary syndrome (PCOS), a disorder of androgen excess, higher testosterone levels are associated with increased adiposity.¹¹⁶ However, the relation between testosterone and adiposity in women without PCOS remains unclear. In one study, postmenopausal women treated with combined estrogen and testosterone therapy showed an increase in lean mass and a greater reduction in whole-body fat mass compared to those receiving estrogen alone.¹¹⁷ Similarly, administration of nandrolone, an anabolic steroid, in women with obesity increased lean mass and reduced whole-body fat mass.¹¹⁸ In contrast, the administration of graded doses of testosterone with estradiol to menopausal women with low testosterone produced no significant effects on total or visceral fat mass.¹¹⁹ Since testosterone dose-dependently increases skeletal muscle mass and strength in testosterone-deficient women,^117–121 these findings suggest that in women, as in men, testosterone’s ability to reduce fat mass depends partly on its capacity to increase lean mass.

In women with PCOS, elevated testosterone is associated with insulin resistance; whether this applies to women without PCOS remains unclear. The metabolic effects of testosterone in women vary by dose, route, genetic background, and clinical context. In women with hypopituitarism or heart failure, a low dose of testosterone improved insulin sensitivity.^122,123 In contrast, another study in postmenopausal women reported mild insulin resistance with testosterone therapy.¹²⁴

In clinical studies of androgen-deficient women—due to hypopituitarism, oophorectomy, or menopause—testosterone replacement enhanced estradiol’s effects to increase bone mineral density.^{118,121,125–127} However, the relative contribution of aromatization to estradiol versus direct AR action remains to be fully elucidated. Still, AR signaling likely contributes to testosterone’s effects on bone health. Female mice lacking AR exhibit reduced trabecular bone mass.¹²⁸

In women, androgen excess is associated with elevated CVD risk. This includes women with polycystic ovary syndrome (PCOS), those receiving high-dose testosterone after menopause, and older women with elevated endogenous testosterone.^129–131 Conversely, low testosterone levels in women are associated with atherosclerosis, CVD, and increased all-cause mortality.^132–135 Testosterone promoted arterial vasodilation in postmenopausal women using estrogen therapy.¹³⁶ In surgical menopause, testosterone combined with estrogen reduced apoCIII levels in VLDL-C and LDL-C, a marker of CVD risk.¹³⁷ Testosterone co-administration also attenuated estradiol-induced CRP elevation in postmenopausal women.¹³⁸ These findings suggest an optimal physiological window for testosterone in women, as both high and low levels appear deleterious.¹³⁹

Women develop atherosclerosis at an older age than men; this sex difference is partly attributable to estradiol protection in reproductive-aged women.¹⁴⁰ LDL-C uptake by macrophages in the arterial wall initiates atherogenesis.¹⁴¹ In vitro, DHT increases cholesterol loading and atherogenic gene expression in male, but not female, macrophages.^142,143 In vivo, testosterone reduced atherosclerosis in apoE^−/− female mice but increased it in males; however, AR-deficient female apoE^−/− mice developed extensive atherosclerosis, indicating a protective AR role in females.^144,145

In short-term trials, testosterone treatment in women with heart failure improved lean mass, muscle strength, and aerobic capacity without noticeable effects on cardiac function.¹⁰¹

In conclusion, a physiological window of testosterone seems to amplify the beneficial effects of estradiol on women’s metabolic health, likely via direct AR actions, although the contribution of aromatization to estradiol cannot be excluded.

Conclusion and future perspectives

Testosterone exerts pleiotropic effects on cardiometabolic physiology. Testosterone and its metabolites act on ERs and ARs expressed in endothelial cells, VSMCs, and cardiomyocytes, modulating vascular tone, endothelial function, lipid handling, and myocardial performance. Testosterone also directly bind calcium channels in VSMCs producing vasorelaxation independent of AR or ERs. Clinically, these actions may contribute to improvements in arterial stiffness, endothelial-dependent vasodilation, and exercise capacity observed in testosterone-treated subjects.

Testosterone actions through ERs and AR also regulates key metabolic tissues—including skeletal muscle, adipose, liver, and pancreatic β-cells—where it enhances muscle mass and bioenergetics, insulin sensitivity and lipid oxidation, reduces visceral adiposity and cholesterol export, and enhances β-cell function. Testosterone also signals in macrophages to suppress chronic low-grade inflammation. These metabolic and immunologic effects may collectively reduce cardiometabolic risk over time.

The integrated in vivo effects of testosterone are context-dependent, influenced by dose, treatment duration, age, sex, and underlying metabolic status, which explains variability in clinical outcomes. While recent RCTs have confirmed the cardiometabolic safety and efficacy of testosterone therapy in hypogonadal men, the mechanistic basis—particularly the cell-specific, genomic and non-genomic signaling through AR and ERs in metabolic, immune, and vascular cells—remains incompletely defined. Elucidating these pathways offers a roadmap to developing receptor- and tissue-selective androgen-based therapies that preserve metabolic and vascular benefits while minimizing risk. Such an approach could enable precision hormone therapy targeting age-related cardiometabolic disease in both men and women.