Testosterone Decline in Men and Women: The Longevity Case for Hormonal Optimisation

Introduction: Rethinking Testosterone

When most people hear the word testosterone, they think of muscle, libido, and masculinity. This reductive framing has done enormous harm — not only by trivialising a hormone of extraordinary systemic importance, but by rendering women almost entirely invisible in the clinical conversation. In my practice, I encounter this bias constantly: men who have been told their testosterone is "within normal range" and dismissed, despite a constellation of symptoms that are eroding their quality of life; and women who are never tested at all, whose fatigue, brain fog, loss of drive, and diminished bone density are attributed to stress, perimenopause, or depression — when the root cause is hormonal depletion that a simple blood test would reveal.

Testosterone is not a sex hormone. It is a pleiotropic signalling molecule — one that acts on receptors distributed throughout virtually every organ system: the brain, the heart, skeletal muscle, bone, adipose tissue, and the vasculature. Its decline is not a trivial accompaniment to normal ageing. It is, I would argue, one of the central biological mechanisms through which ageing accelerates — and one of the most actionable.

This article presents the evidence base for testosterone as a longevity hormone: the physiology of its decline, the downstream consequences across organ systems, the diagnostic framework that functional medicine employs to identify those most at risk, and the clinical case for optimisation in both men and women.

Testosterone Physiology: What the Textbooks Miss

Synthesis and Regulation

In men, approximately 95% of circulating testosterone is produced by Leydig cells in the testes under stimulation from luteinising hormone (LH), itself regulated by pulsatile gonadotropin-releasing hormone (GnRH) from the hypothalamus. In women, testosterone is produced in roughly equal measure by the ovaries and the adrenal glands, with additional peripheral conversion from androstenedione in adipose tissue. Far from being an exclusively male hormone, testosterone is present in women at concentrations that, while lower in absolute terms, exert proportionally significant effects given the sensitivity of female androgen receptors [4].

In both sexes, the majority of circulating testosterone — roughly 60–80% — is bound to sex hormone-binding globulin (SHBG), a hepatically synthesised glycoprotein with high affinity for androgens and oestrogens. A further 20–38% is loosely bound to albumin. Only 1–3% circulates as free testosterone — the fraction available for cellular uptake and receptor binding. This distinction is clinically decisive: a man or woman with a total testosterone at the lower end of the reference range but elevated SHBG may have a free testosterone that is frankly deficient, yet standard laboratory reporting will categorise the result as normal.

The SHBG Problem

SHBG rises with age, with oestrogen exposure, with hyperthyroidism, with hepatic disease, and — critically — with insulin sensitivity. Conversely, it falls with obesity, hypothyroidism, hyperinsulinaemia, and glucocorticoid excess. This means that two individuals with identical total testosterone may have radically different free testosterone levels depending on their metabolic state. In clinical practice, I calculate free testosterone using the Vermeulen formula when direct free testosterone assays are unavailable or unreliable, and I always interpret total testosterone alongside SHBG before drawing any conclusion about hormonal sufficiency.

Testosterone Beyond the Gonads

Androgen receptors (ARs) are expressed in the hippocampus, prefrontal cortex, amygdala, hypothalamus, cardiac myocytes, vascular smooth muscle, osteoblasts and osteoclasts, skeletal muscle, and adipose tissue [7]. Testosterone's systemic roles include:

• Neurological: Neuroprotection, synaptic plasticity, regulation of dopaminergic and serotonergic tone, spatial cognition, verbal memory in women
• Cardiovascular: Vasodilation via nitric oxide pathways, regulation of erythropoiesis, insulin sensitisation, anti-inflammatory signalling
• Musculoskeletal: Promotion of myofibrillar protein synthesis, inhibition of muscle protein breakdown, stimulation of osteoblast activity and bone mineralisation
• Metabolic: Enhancement of insulin sensitivity, reduction of visceral adiposity, favourable modulation of lipid metabolism
• Psychological: Regulation of mood, motivation, assertiveness, and stress resilience

The Timeline of Decline: Earlier Than We Think

Men: A Cohort Effect, Not Just Ageing

In men, testosterone peaks in the late teens to early twenties and then declines at approximately 1–2% per year from the third decade onwards [1]. By age 70, average total testosterone is roughly 35% lower than at peak. But the landmark paper by Travison and colleagues, published in the Journal of Clinical Endocrinology and Metabolism in 2007, revealed something more alarming: a population-level secular decline independent of age [1]. Men born in the 1930s had higher testosterone at any given age than men born in the 1960s — suggesting that environmental, dietary, and lifestyle factors are driving a civilisational erosion of androgen status that cannot be explained by ageing alone.

Candidate drivers of this secular decline include rising rates of obesity and insulin resistance (which suppress Leydig cell function and elevate SHBG indirectly via inflammatory pathways), ubiquitous exposure to endocrine-disrupting chemicals (phthalates, bisphenol A, pesticides), chronic sleep deprivation (testosterone synthesis is nocturnal and sleep-dependent), sedentary behaviour, and the dysregulation of the hypothalamic-pituitary-gonadal (HPG) axis by chronic psychological stress and elevated cortisol [1, 7].

Women: The Silent Decline

In women, the androgen story is written across a longer arc and is — scandalously — even less well understood in clinical practice. Testosterone begins to fall in women from the late twenties onwards, well before the perimenopausal transition. By the time a woman reaches her forties, her testosterone may be 50% lower than it was at 20. The adrenopause — a decline in adrenal androgen secretion (particularly DHEA and DHEA-S) — compounds ovarian androgen decline and typically precedes the menopause by a decade [4].

The Study of Women's Health Across the Nation (SWAN), a major longitudinal cohort study, has provided much of our understanding of sex hormone trajectories in midlife women. SWAN data demonstrate that free testosterone, bioavailable testosterone, and DHEA-S all decline significantly across the menopausal transition, while SHBG increases — producing a double suppression of free androgen availability [10]. Surgically induced menopause (bilateral oophorectomy) causes an abrupt, severe reduction in testosterone that is frequently undertreated.

Davis and colleagues' landmark systematic review in The Lancet Diabetes & Endocrinology (2015) established that testosterone is clinically significant in women across multiple domains, including sexual function, mood, energy, lean body mass, and bone mineral density — yet prescribing guidelines remain inconsistent, and many clinicians are still trained to believe that testosterone replacement in women is experimental or without evidence [4].

Symptoms Beyond Libido: The Systemic Picture

Cognitive Function

The evidence linking testosterone to cognitive function is compelling in both sexes. Androgen receptors are densely expressed in the hippocampus and prefrontal cortex — the anatomical substrates of memory consolidation, executive function, and working memory. In men, low testosterone is associated with worse performance on tests of spatial cognition, verbal fluency, and processing speed [7]. In women, endogenous testosterone correlates positively with verbal memory and cognitive flexibility, and the precipitous androgen drop following surgical menopause has been associated with accelerated cognitive decline [4].

Testosterone's neuroprotective mechanisms include promotion of neuronal survival, reduction of amyloid-beta accumulation (relevant to Alzheimer's pathology), and modulation of the cholinergic and dopaminergic systems that underpin attention and memory. I regard the cognitive domain as one of the most compelling — and least discussed — arguments for hormonal optimisation in the preventive longevity context.

Cardiovascular Risk

For decades, a misplaced concern that testosterone replacement would increase cardiovascular risk — based on flawed early studies and a naive extrapolation from anabolic steroid abuse — paralysed clinical prescribing. The TRAVERSE trial, the largest randomised controlled trial of testosterone replacement therapy ever conducted (n = 5,246 men with hypogonadism and established or high cardiovascular risk), reported in The New England Journal of Medicine in 2023, definitively resolved this question [3]. Testosterone replacement therapy was non-inferior to placebo for the primary composite cardiovascular endpoint of death from cardiovascular causes, non-fatal myocardial infarction, and non-fatal stroke. This finding, rigorously adjudicated in a pre-specified non-inferiority design, should have changed clinical practice immediately.

Mechanistically, testosterone's cardiovascular protection operates through multiple pathways: it promotes vasodilation via endothelial nitric oxide synthase (eNOS) activation, reduces pro-inflammatory cytokines (IL-6, TNF-alpha), improves insulin sensitivity and glycaemic control, reduces visceral adiposity, and supports favourable haematological and lipid profiles at physiological doses [3, 7]. Laughlin and colleagues, analysing data from the Rancho Bernardo cohort, demonstrated that men in the lowest testosterone quartile had a 40% higher age-adjusted cardiovascular mortality compared to those in the highest quartile [6].

Bone Mineral Density

Testosterone contributes directly to bone mineralisation through androgen receptors on osteoblasts and indirectly through its aromatisation to oestradiol, which is the primary driver of bone protective effects in both sexes. In men, hypogonadism is one of the most common causes of secondary osteoporosis — a connection that is systematically underappreciated in clinical practice, where osteoporosis is often framed as a women's disease [7]. In women, declining androgen levels in the perimenopausal decade contribute significantly to the accelerated bone loss that precedes fracture risk in later life. SWAN data confirm that lower free testosterone in perimenopausal women is independently associated with lower bone mineral density at the lumbar spine and femoral neck [10].

Metabolic Health and Body Composition

The relationship between testosterone and metabolic health is bidirectional and self-reinforcing. Low testosterone promotes visceral adiposity; visceral adipose tissue expresses high levels of aromatase, which converts testosterone to oestradiol, further suppressing testosterone via HPG axis feedback. This vicious cycle underlies the common clinical picture of the overweight middle-aged man — or woman — with low energy, central adiposity, insulin resistance, and hormonal depletion, each element feeding the others [7].

Testosterone enhances insulin-stimulated glucose uptake in skeletal muscle, reduces hepatic fat accumulation, and promotes the differentiation of mesenchymal stem cells towards muscle rather than adipose lineage. In men with hypogonadism, testosterone replacement consistently improves lean body mass, reduces fat mass, and improves insulin sensitivity and HbA1c — with effects comparable to first-line pharmacological interventions for metabolic syndrome [2].

Muscle Mass and Physical Resilience

Sarcopenia — the age-related loss of muscle mass and function — is one of the strongest predictors of frailty, falls, disability, and all-cause mortality in older adults. Testosterone is the primary anabolic signal for skeletal muscle. Bhasin and colleagues' foundational dose-response study demonstrated a linear relationship between testosterone dose and lean body mass accretion and leg press strength in healthy men, establishing the causal role of testosterone in muscle anabolism [2]. In older hypogonadal men, testosterone replacement attenuates the decline in lean mass and preserves physical function in a way that exercise alone cannot fully replicate in the context of hormonal deficiency.

Mood, Motivation, and Psychological Wellbeing

The dopaminergic and serotonergic systems — the neurochemical substrates of motivation, reward, and emotional regulation — are potently modulated by testosterone. Hypogonadal men consistently exhibit higher rates of depression, anhedonia, and irritability, and multiple randomised trials demonstrate that testosterone replacement significantly improves depressive symptoms compared to placebo, particularly in men with borderline-to-low testosterone [7]. In women, androgen insufficiency presents as a loss of drive, blunted affect, reduced assertiveness, and emotional flatness that is frequently misattributed to primary psychiatric disease. This is not depression in the conventional sense — it is a hormonal signal, and it deserves a hormonal response.

Testosterone and All-Cause Mortality: The Epidemiological Case

The association between low testosterone and premature death is among the most consistent findings in the epidemiological literature on male ageing. Shores and colleagues, in a retrospective cohort study of male veterans published in Archives of Internal Medicine (2006), found that men with low testosterone had a significantly higher all-cause mortality over a median follow-up of 8.2 years, with a hazard ratio of 1.88 compared to those with normal testosterone — a near-doubling of mortality risk [5]. This association persisted after adjustment for age, medical comorbidities, BMI, and medication use.

Laughlin and colleagues, using data from the prospective Rancho Bernardo Study, reported similar findings in a community-dwelling cohort: low bioavailable testosterone was independently associated with increased all-cause and cardiovascular mortality in older men, with the lowest quartile of testosterone conferring a 40% excess mortality risk after multivariate adjustment [6]. The Framingham Heart Study data, derived from one of the most rigorously characterised cohorts in cardiovascular epidemiology, similarly demonstrate that low testosterone in men is associated with increased prevalence of metabolic syndrome, greater visceral adiposity, and worse cardiometabolic risk profiles — findings that contextualise the mortality data within a mechanistic framework [9].

The direction of causality is not settled by observational data alone — sick men have low testosterone, and low testosterone makes men sicker — but the biological plausibility of testosterone's protective effects, combined with the TRAVERSE trial's cardiovascular safety data and multiple intervention studies showing favourable metabolic effects of replacement, strengthens the case considerably.

The Diagnostic Gap: Subclinical Hypogonadism

The Reference Range Problem

Standard laboratory reference ranges for total testosterone are typically constructed from population distributions — they define "normal" as the central 95% of values observed in a mixed-age sample. This means that a 55-year-old man whose testosterone has declined to the same level as a healthy 75-year-old may be told his results are "within range." The range is descriptive, not prescriptive. It tells us what is common; it tells us nothing about what is optimal.

In functional medicine, we distinguish between the reference range and the optimal range. For total testosterone in men, the conventional lower limit of normal is approximately 300 ng/dL (10.4 nmol/L) in most laboratory systems. But multiple studies suggest that symptoms of androgen deficiency — fatigue, depression, reduced lean mass, metabolic dysfunction — begin to manifest at levels between 300–400 ng/dL, and that the threshold for adverse health outcomes may be higher still [2, 7]. The concept of subclinical hypogonadism — symptomatic deficiency in the context of borderline-normal biochemistry — is not adequately captured by current diagnostic frameworks.

The Female Diagnostic Desert

For women, the situation is still more problematic. There is no universally agreed reference range for testosterone in women. Laboratory ranges vary widely between institutions, many reference intervals are derived from small cohorts of questionable representativeness, and the menopausal stage at which the sample was drawn is often unspecified. The Global Consensus Position Statement on the Use of Testosterone Therapy for Women (2019) acknowledged that validated population-based reference ranges for total and free testosterone in women across the lifespan are urgently needed [4, 13].

In practice, this means that a perimenopausal woman with free testosterone at the very bottom of the quoted reference range — the same concentration as a post-menopausal woman twenty years her senior — may receive a normal report and no further investigation. The diagnostic infrastructure simply does not exist to identify her deficiency, because no one has agreed what optimal looks like in her demographic.

The Full Panel: What I Assess

In my practice, hormonal evaluation for both men and women includes:

• Total testosterone — morning fasting sample (diurnal peak is 07:00–10:00 in men)
• SHBG — essential for interpreting free testosterone
• Free testosterone — calculated via Vermeulen formula or measured by equilibrium dialysis
• DHEA-S — adrenal androgen precursor, declines with age and stress; reflects adrenal reserve
• LH and FSH — to distinguish primary (testicular/ovarian) from secondary (hypothalamic/pituitary) hypogonadism
• Oestradiol — particularly in men, where aromatisation of testosterone to oestradiol is a frequent cause of symptoms and requires monitoring during replacement therapy
• Prolactin — to exclude pituitary pathology in cases of secondary hypogonadism
• Full metabolic panel, HbA1c, lipids — because testosterone status cannot be interpreted in isolation from metabolic context

The clinical picture — the patient's symptoms, their history, their metabolic phenotype — always takes precedence over a single laboratory value. Biochemistry informs; it does not dictate.

Who Is a Candidate for Hormonal Optimisation?

Men

Male candidates for testosterone optimisation include those presenting with the classic symptom cluster of androgen deficiency — reduced energy, impaired concentration, low mood, reduced libido, loss of morning erections, declining muscle mass and strength, increased visceral fat, and worsening glycaemic control — in the context of biochemistry demonstrating low or low-normal total testosterone and/or reduced free testosterone. Age alone is not a criterion; nor is it a contraindication. The clinical evaluation must also exclude secondary causes of low testosterone (severe obesity, obstructive sleep apnoea, pituitary pathology, opioid use, anabolic steroid misuse) that are amenable to primary treatment.

Absolute contraindications to testosterone replacement in men include active prostate cancer, breast cancer, severe untreated erythrocytosis (haematocrit >54%), and uncontrolled heart failure. Fertility preservation in younger men requires alternative approaches (clomiphene citrate or gonadotrophin therapy), since exogenous testosterone suppresses spermatogenesis via HPG axis negative feedback [2, 3].

Women

For women, the strongest evidence base — and the current international consensus — supports testosterone therapy in postmenopausal women with hypoactive sexual desire disorder (HSDD) that is not attributable to other treatable causes [4, 13]. However, in a longevity and functional medicine context, the indication is broader: peri- and post-menopausal women with fatigue, cognitive slowing, loss of lean mass, reduced bone mineral density, mood changes, and biochemical evidence of androgen insufficiency are appropriate candidates for a therapeutic trial.

The principle of physiological dosing is paramount in women. Testosterone therapy in women should target serum concentrations in the upper end of the premenopausal reference range — not supraphysiological levels. Transdermal formulations (gels, creams, or patches) are preferred to injectable preparations to maintain steady-state levels and minimise androgenic side effects. Monitoring at 3 and 6 months, with dose titration guided by symptom response and biochemistry, is standard practice [4, 13].

The Functional Medicine Approach: Optimise, Don't Just Replace

Before prescribing testosterone, I always work to address the modifiable drivers of testosterone decline. The most potent interventions include:

• Sleep optimisation: 70–80% of daily testosterone release occurs during slow-wave sleep. A man sleeping 5 hours per night may have testosterone levels 10–15% lower than when sleeping 8 hours — a significant deficit that is entirely reversible.
• Resistance training: The most powerful lifestyle stimulus for endogenous testosterone synthesis. Compound, multi-joint movements at moderate-to-high intensity, performed 3–4 times per week, reliably elevate resting testosterone and improve receptor sensitivity.
• Nutritional adequacy: Zinc, magnesium, and vitamin D are cofactors in testosterone biosynthesis. Chronic caloric restriction and very low-fat diets suppress androgen synthesis. Adequate dietary fat — including saturated fat from whole food sources — is necessary for steroidogenesis.
• Stress management and cortisol reduction: Chronic HPA axis activation suppresses GnRH pulsatility and Leydig cell function. Cortisol is the endocrine system's veto on reproductive function.
• Body composition: Reducing visceral adiposity reduces aromatase activity, lowers SHBG suppression by insulin, and directly increases free testosterone. Weight loss in obese men is one of the most effective interventions for raising testosterone.
• Endocrine disruptor reduction: Minimising exposure to plasticisers, pesticides, and parabens reduces xenoestrogen burden and its suppressive effects on HPG axis function.

Where lifestyle and nutritional optimisation are insufficient — as is frequently the case in patients with significant androgen deficiency, particularly those over 50 — testosterone replacement therapy is the appropriate next step. The two approaches are not mutually exclusive; they are complementary and synergistic.

Delivery Methods and Monitoring

In men, testosterone replacement is most commonly delivered as transdermal gel or cream (daily application), long-acting injectable testosterone undecanoate (12-weekly), or shorter-acting injectable testosterone enanthate/cypionate (1–2 weekly). Each has distinct pharmacokinetic profiles, and the choice should be guided by the patient's preference, lifestyle, and clinical characteristics [2, 3].

Monitoring during testosterone replacement therapy includes: serum testosterone (targeting the mid-to-upper physiological range), haematocrit (erythrocytosis is the most common adverse effect, occurring in up to 5–6% of men in the TRAVERSE trial), PSA in men over 40 or those with prostate risk factors, and symptom review. Oestradiol monitoring is important in men prone to aromatisation, particularly those with higher adiposity.

Testosterone as a Longevity Hormone: Dr. Sadaf's Perspective

I want to close this article with a position statement, because I think this field demands intellectual honesty rather than institutional caution.

The evidence that testosterone is a longevity-relevant hormone — not merely a sex hormone — is, in my assessment, now overwhelming. The epidemiological data showing that low testosterone predicts all-cause mortality [5, 6], the mechanistic data showing its protective effects across cardiovascular, neurological, musculoskeletal, and metabolic domains, and the pivotal TRAVERSE trial resolving the cardiovascular safety question [3] collectively constitute an evidence base that should shift clinical practice substantially.

We are not — and I want to be explicit about this — proposing pharmacological superphysiological testosterone. We are proposing the restoration of physiological concentrations that declining endogenous synthesis can no longer sustain. The argument is analogous to thyroid replacement: we do not debate whether replacing deficient thyroid hormone is "natural" — we replace it because its absence causes systemic harm. The same reasoning applies to testosterone.

What prevents wider adoption is not the evidence — it is the cultural scaffolding around the hormone. Testosterone has been politicised, weaponised, and caricatured in equal measure. It has been portrayed as the hormone of excess: of aggression, of abuse, of unfair advantage. This caricature is the anabolic steroid talking. Physiological testosterone — at concentrations commensurate with a healthy 30-year-old — is a profound biological regulator of human vitality and resilience. Allowing it to decline without investigation or intervention, while prescribing statins for the cardiovascular consequences and antidepressants for the neurological ones, is not evidence-based medicine. It is compartmentalised medicine — and it costs patients decades of wellbeing.

In a truly preventive longevity practice, the hormonal milieu is not an afterthought. It is foundational. Testosterone — alongside oestradiol, progesterone, thyroid hormones, DHEA, and growth hormone — forms the endocrine architecture within which every other longevity intervention operates. Optimise the architecture, and everything else works better.