Testosterone Deficiency Reassessed: What Modern Evidence Says About Diagnosis and Replacement Therapy

Testosterone deficiency, also known as male hypogonadism, has long occupied an uneasy position in medicine: a genuine clinical syndrome for some men, a contested label for others, and a marketing opportunity for the supplement industry. A comprehensive review published in Nature Reviews Urology now synthesizes the contemporary evidence base, offering clinicians a carefully calibrated update on how the condition should be defined, diagnosed and treated. The review, led by Thomas Thompson and Mohamed Mubarak of the Manchester Andrology Research Collaborative together with T. Hugh Jones, Ian Pearce and Vaibhav Modgil, arrives at a nuanced conclusion: testosterone replacement therapy delivers real, measurable benefits in appropriately selected men, while contemporary trial data do not support the fears of prostate cancer and cardiovascular harm that have shadowed the field for two decades.

The clinical syndrome at the heart of the debate is defined by the coexistence of characteristic symptoms with persistently low serum testosterone concentrations. That dual requirement matters, because testosterone levels alone tell an incomplete story. The condition exerts wide-ranging effects across the male body, influencing sexual function, metabolic health, bone integrity, mood, haematopoiesis and overall quality of life. The authors classify hypogonadism into three broad categories: primary hypogonadism, arising from testicular failure; secondary hypogonadism, driven by dysfunction of the hypothalamic–pituitary axis; and functional hypogonadism, a form commonly associated with ageing, obesity and chronic systemic disease. The functional category has attracted growing attention because it blurs the line between endocrine disease and the physiological consequences of modern lifestyles, and because it may be partially reversible when the underlying drivers are addressed.

The mechanistic interplay between adiposity and testosterone is particularly striking. Adipose tissue expresses aromatase, the enzyme that converts androgens into oestrogens, so expanding fat mass both consumes testosterone and suppresses the hypothalamic–pituitary–gonadal axis through feedback and inflammatory adipocytokine signalling. This hypogonadal–obesity–adipocytokine hypothesis, illustrated in the review, describes a self-reinforcing cycle: low testosterone promotes visceral fat accumulation and insulin resistance, which in turn further suppress testosterone production. Recent work cited in the review has documented altered expression of aromatase and oestrogen receptors in adipose tissue from men with obesity and type 2 diabetes, providing a molecular substrate for the clinical observations. Breaking this cycle, whether through weight loss, pharmacotherapy or testosterone replacement itself, is therefore a central therapeutic goal.

Diagnosis remains the discipline’s most contested territory, and the review is emphatic about rigour. Confirmation requires both clinical features and biochemical evidence, typically obtained through morning serum testosterone measurements, because testosterone follows a diurnal rhythm that flattens with age and is blunted by poor sleep and illness. When values fall into a borderline zone, or when symptoms and total testosterone concentrations appear discordant, measurement of sex hormone-binding globulin and calculated free testosterone can provide additional clarity, since a substantial fraction of circulating testosterone is bound and biologically inactive. The authors highlight recent joint position statements from the Society for Endocrinology and the Association for Clinical Biochemistry and Laboratory Medicine aimed at standardizing biochemical confirmation, an attempt to reduce the diagnostic variability that has plagued the field. Symptom questionnaires such as the Aging Males’ Symptoms scale and the quantitative ADAM instrument can support, but never replace, laboratory confirmation.

Once the diagnosis is secure, the evidence for treatment is substantial. Meta-analyses of randomized trials report consistent improvements in sexual desire and erectile function, with testosterone therapy also enhancing the response to phosphodiesterase 5 inhibitors in men who previously failed to respond. The large TRAVERSE trial programme has added high-quality data on mood, showing improvement in depressive syndromes among hypogonadal men, while earlier trials demonstrated benefits in energy and health-related quality of life. Haematological effects are equally well documented: testosterone stimulates erythropoiesis, and randomized studies have shown that replacement therapy corrects anaemia in hypogonadal men, a finding with potential relevance for older men whose fatigue and functional decline are partly attributable to low red cell mass.

Metabolic and skeletal outcomes form a second pillar of benefit. Trials in men with type 2 diabetes and metabolic syndrome, including the TIMES2 study and the T4DM trial, indicate that testosterone replacement improves insulin resistance, glycaemic control and body composition, reducing fat mass while increasing lean mass. Registry data suggest that long-term therapy can even be associated with remission of type 2 diabetes in some hypogonadal men. On bone, testosterone is aromatized to oestradiol in skeletal tissue, and both sex steroids contribute to bone maintenance. Randomized trials have shown increases in bone mineral density and improvements in volumetric bone strength, and a 2024 analysis in the New England Journal of Medicine reported that testosterone treatment reduced fracture risk in men with hypogonadism, converting a long-standing surrogate outcome into a hard clinical endpoint.

Safety has been the field’s defining controversy. The historical fear that testosterone fuels prostate cancer stemmed from the observation, dating to Huggins and Hodges in 1941, that androgen deprivation shrinks metastatic disease. The modern saturation model proposes instead that androgen-dependent growth plateaus at low concentrations, so raising testosterone from deficient to normal levels has limited proliferative effect. Contemporary data reviewed by the authors, including meta-analyses of prostate cancer incidence in treated men and studies of therapy in prostate cancer survivors after definitive local treatment, do not demonstrate increased oncological risk when monitoring is appropriate. Cardiovascular safety has followed a similar arc from alarm to reassurance: early observational studies suggesting excess myocardial infarction were followed by the TRAVERSE trial, published in 2023, which found that testosterone replacement did not increase major adverse cardiovascular events in middle-aged and older men, and by subsequent European expert panel position statements concluding that appropriately monitored therapy is cardiovascularly safe.

Not all risks can be dismissed, however, and the review is candid about the caveats. Testosterone therapy suppresses spermatogenesis through inhibition of gonadotropin secretion, making it inappropriate for men who wish to preserve fertility; alternatives such as selective oestrogen receptor modulators, aromatase inhibitors, human chorionic gonadotropin and, in trials of male obesity-related hypogonadism, the aromatase inhibitor leflutrozole can raise testosterone while maintaining or restoring sperm production. Erythrocytosis remains the most common laboratory adverse effect requiring dose adjustment, and testosterone therapy can worsen obstructive sleep apnoea, which should be assessed and treated before and during treatment. The authors also flag a practical laboratory pitfall: contamination of the venipuncture site with topical testosterone gel can produce falsely elevated readings, a reminder that even the diagnostic pathway demands attention to detail.

The therapeutic arsenal has expanded considerably. Transdermal gels and solutions remain the most widely used formulations, offering physiological absorption but requiring daily application and posing a theoretical risk of transfer to partners and children. Newer oral testosterone undecanoate formulations, absorbed via the lymphatic system when taken with food, avoid the hepatotoxicity that doomed earlier oral preparations and have shown efficacy across body mass index ranges in phase III trials. Long-acting intramuscular testosterone undecanoate injections provide stable levels for ten to fourteen weeks, while subcutaneous enanthate auto-injectors and subdermal pellets serve patients who prefer less frequent administration. Intranasal gel offers rapid achievement of steady-state levels and, notably, has been shown in trials to preserve semen parameters better than other routes, an option for younger men with fertility concerns. Patient preference, adherence data and pharmacokinetic profiles all inform the choice, and the review stresses that individualized selection, careful counselling and structured monitoring of testosterone levels, haematocrit and prostate-specific antigen are essential to optimize outcomes.

The broader significance of this update lies in its recalibration of a polarized debate. The evidence now supports a middle path: testosterone deficiency is a real and treatable syndrome with demonstrable benefits across sexual, metabolic, skeletal, haematological and psychological domains, but it is neither a panacea for ageing nor a condition to be diagnosed from a single afternoon blood test. Regulatory bodies, including the European Medicines Agency and the US Food and Drug Administration, have moved toward positions consistent with the trial evidence, though the FDA continues to require labelling about possible cardiovascular risk in age-related use. For the millions of men worldwide whose symptoms, metabolic disease and bone fragility intersect with low testosterone, the message from this authoritative synthesis is measured but clear: with rigorous diagnosis, appropriate patient selection and disciplined monitoring, testosterone replacement therapy has earned its place in modern endocrine practice.

Subject of Research: Diagnosis, clinical benefits and safety of testosterone replacement therapy in male hypogonadism

Article Title: Update on testosterone deficiency and therapeutic options for testosterone replacement therapy

Article References: Thompson, T., Mubarak, M., Jones, T. H., Pearce, I., & Modgil, V. (2026). Update on testosterone deficiency and therapeutic options for testosterone replacement therapy. Nature Reviews Urology. https://doi.org/10.1038/s41585-026-01190-3

Image Credits: AI Generated

DOI: 10.1038/s41585-026-01190-3

Keywords: testosterone deficiency, male hypogonadism, testosterone replacement therapy, hypothalamic-pituitary-gonadal axis, obesity, type 2 diabetes, bone mineral density, cardiovascular safety, prostate cancer, TRAVERSE trial, free testosterone, hypogonadism diagnosis