Testosterone Therapy

The cardiovascular fear that defined twenty years of testosterone prescribing has collapsed under one large trial, but a 2026 genetic study and Wilder's Law of Initial Value reframe what's left: testosterone is a metabolic rescue for deficient men with cardiometabolic disease, not a longevity drug for healthy ones.

Testosterone replacement therapy (TRT) has lived under a heavier regulatory cloud than almost any drug in routine internal medicine — a 2015 FDA boxed cardiovascular warning, retrospective signals of acute harm, and decades of observational data linking low endogenous testosterone to early death. The 2023–2026 evidence has reorganized the picture. The TRAVERSE trial settled the cardiovascular question prospectively. A 737-man treatment registry showed an extraordinary mortality reduction in the men with the most to lose. A genetic study using lifelong-exposure data flagged a subtler hypertensive risk that clinical trials are too short to see. None of these alone tells the whole story; together they sketch a calibrated answer to who actually benefits.

What testosterone does, and what "low" means

In adult men testosterone and its metabolites (dihydrotestosterone, estradiol) drive muscle protein synthesis, suppress visceral adiposity, maintain bone mineral density, stimulate erythropoiesis, and support libido, executive cognition, and mood.^[1] Endogenous production declines about 1.6% per year from the mid-thirties onward; population estimates put 20% of men over 60, 30% over 70, and 50% over 80 below the conventional 300 ng/dL threshold for clinical hypogonadism.^[2]

Two thresholds are in active use:

AUA: total testosterone <300 ng/dL on two separate morning draws plus symptoms.^[3]
Endocrine Society: <264 ng/dL — slightly stricter, more conservative.^[4]

Symptoms of late-onset hypogonadism — fatigue, low libido, poor recovery, mood flatness, lost lean mass — are nonspecific and overlap with poor sleep, alcohol, untreated OSA, depression, overtraining, and several common medications. Both societies require lifestyle correction first before pharmacological replacement.

Lifestyle first — endogenous rescue (Strong)

Before any prescription, the modern guideline path is correction of the inputs that suppress endogenous production:

Sleep. Restricting sleep by even one hour per night relative to baseline drops serum testosterone by 10–15% in healthy young men.^[5] Untreated obstructive sleep apnea is the single largest reversible suppressor — and OSA causes the same fatigue/low-libido phenotype TRT is often prescribed for. Screen for OSA before pursuing TRT (insomnia-treatment, sleep-breathing).
Exercise. 150–300 minutes/week of moderate aerobic activity combined with resistance training raises endogenous testosterone, reduces visceral fat, and improves sexual function — both AUA and international urological guidelines list it as a mandatory first-line intervention.^[6] See resistance-training.
Stress and alcohol. Chronic hypercortisolemia directly suppresses the hypothalamic-pituitary-gonadal axis; high-occupational-stress cohorts show measurably lower serum testosterone.^[7] Heavy alcohol use compounds the suppression.
Endocrine-disrupting chemicals. Phthalates, bisphenols, and parabens depress sex hormone-binding globulin (SHBG) and disrupt androgen signalling; the IRECO trial showed urinary biomarkers drop within five days of personal-care substitution. See environmental-toxins.
Varicocele repair (where present) durably raises mean total testosterone — a structural rather than pharmacological fix.

A meaningful fraction of men presenting as "low T" recover into the normal range after these inputs are corrected. About 25% of men start TRT without comprehensive baseline lab evaluation — a reckless starting point given how many of these substitutes for diagnosis.^[8]

Cardiovascular safety — what TRAVERSE settled (Strong)

The defining trial of the field is TRAVERSE — an FDA-mandated, event-driven, double-blind RCT of 5,204 men aged 45–80 with documented hypogonadism (two morning T <300 ng/dL plus symptoms) plus either established cardiovascular disease or elevated CV risk, randomized to daily transdermal testosterone gel vs placebo for a mean 21.7 months of treatment with 33 months follow-up.^[9]

The primary composite of CV death, non-fatal MI, and non-fatal stroke occurred in 7.0% of the testosterone arm vs 7.3% of placebo — formally non-inferior. There was no excess of prostate cancer or worsening lower urinary tract symptoms.^[10] The FDA removed the boxed cardiovascular warning from testosterone product labels in 2025, replacing it with a TRAVERSE-results disclosure.^[11]

The trial also found a clinically significant 22.5% reduction in progression to clinical diabetes — notable because the dosing was actually conservative: ~25% of treated men never reached the normal physiologic range, and mean T rose only 9.3–12.9 nmol/L.^[12] The metabolic benefit appeared even with under-replacement.

Secondary signals to monitor

TRAVERSE was not unambiguously clean. The testosterone arm showed higher rates of:

Atrial fibrillation — a real arrhythmia signal, important in older men screening for AF.
Acute kidney injury and pulmonary embolism — modest absolute increases, more relevant in patients with baseline risk factors.
Elevated blood pressure — confirmed on ambulatory monitoring; the FDA added a label warning for some testosterone products.

These belong on the consent conversation, not in the headline.

The Mendelian randomization paradox (Moderate)

The opposite-direction finding came from a 2026 Mendelian randomization study using genetic instruments on 425,097 UK Biobank adults of European ancestry, cross-referenced with CAD outcomes from >1.16 million people in CARDIoGRAMplusC4D.^[13] Men with a genetic predisposition to higher lifelong testosterone had a 17% higher CAD risk (OR 1.17, 95% CI 1.07–1.27) — equivalent in absolute terms to a lifetime CAD risk rising from ~7.3% to ~8.5%. The signal was null in women (OR 1.01).^[14] Adjusting for systolic and diastolic blood pressure substantially weakened the association — most of the genetic signal appears to be lifelong hypertensive shear stress, not lipid or inflammatory mechanisms.

The result looks contradictory next to TRAVERSE but isn't. MR captures cumulative exposure from puberty onward; clinical trials capture months-to-years of restoration in older men whose deficiency itself drives visceral adiposity, insulin resistance, and inflammation. Restoring testosterone in a metabolically compromised 60-year-old rescues the dominant short-term killer; lifelong elevated testosterone in an otherwise healthy 30-year-old slowly raises blood pressure for decades.^[15]

The take-home: target the middle of the normal range, not the top. Optimization toward 25-year-old physiology in a 55-year-old buys subjective gains at the cost of decades of higher hypertensive load.

Mortality registry and the Law of Initial Value (Moderate)

A 2024 longitudinal registry of 737 men with adult-onset testosterone deficiency followed for a median of 114 months reported a hazard ratio of 0.23 (95% CI 0.14–0.40) for all-cause mortality with long-acting testosterone undecanoate vs untreated matched controls — independent of age, waist circumference, HbA1c, lipids, BP, smoking, and T2D.^[16]

Stratified analysis is the more honest reading: the survival benefit was concentrated in men with the highest baseline cardiometabolic risk. Relatively healthy hypogonadal men did not show the same magnitude of effect. The authors invoked Wilder's "Law of Initial Value" — the response to a therapy is roughly proportional to how far from baseline the system has drifted.^[17]

This is the same pattern as the Ozempic-class drugs: biggest effect in the sickest patients, smallest in healthy adults seeking optimization. TRT is a metabolic rescue, not a generic life-extension lever.

Cognition and mood (Moderate)

A 2025 systematic review and meta-analysis of 14 trials of androgen replacement in hypogonadal men reported domain-specific improvements:^[18]

Domain	Standardized mean difference	p
Executive function	0.49 (0.37–0.60)	<0.001
Verbal/working memory	0.46 (0.34–0.58)	<0.001
Visuospatial	0.23 (0.15–0.31)	<0.001
Attention	0.22 (0.08–0.35)	0.001

Executive function and memory show the most robust signal — consistent with hippocampal androgen receptor density. Qualitative reviews also report symptom improvement in treatment-resistant depression and mild Alzheimer's.^[19] Despite this, the AUA still advises clinicians to label cognitive benefits as "inconclusive" (Grade B) — a notable institutional lag behind the meta-analytic data.^[20] For a healthy adult, TRT is not first-line for cognitive complaints; the dementia-prevention levers (hearing, BP, exercise, MIND diet, sleep) outperform on absolute terms.

Epigenetic effects (Weak / preliminary)

The TRIIM trial (n=9, men 51–65) used a multi-agent regimen — recombinant human growth hormone, DHEA, and metformin — and reported a 1.5–2.5 year mean regression in epigenetic age across four major clocks over 12 months, plus thymic mass restoration.^[21] The TRIIM-X extension is ongoing.^[22] Testosterone wasn't the principal driver, but the trial established that targeted hormone manipulation can move epigenetic clocks measurably.

In population data, higher male testosterone-to-estradiol ratios link to younger PhenoAge and substantially lower DNA methylation-based PAI-1, a biomarker of thrombosis and senescence — a one-SD increase in total T was associated with a 481 pg/mL drop in DNAm PAI-1.^[23] See hallmarks-of-aging for the underlying altered intercellular communication hallmark.

A controlled longitudinal study of 13 trans men and 13 trans women showed sexually dimorphic effects in the first year of gender-affirming hormone therapy: testosterone in trans men stabilized DunedinPACE (estimate −0.013) but reduced DNA methylation telomere length (estimate −0.057, p=0.037), while estrogen-based regimens accelerated DunedinPACE (+0.057, p=0.002) with slight telomere gains.^[24] Testosterone is a tissue- and sex-specific regulator, not a generic anti-aging molecule.

Female testosterone therapy (Moderate, narrow indication)

Testosterone is a primary female sex steroid; women produce more testosterone than estrogen by mass. The only globally endorsed indication for female testosterone therapy is Hypoactive Sexual Desire Disorder (HSDD) in postmenopausal women.^[25]

Emerging evidence extends the case beyond libido. A 2025 Journal of Personalized Medicine analysis of 332 women aged 27–78 on individualized testosterone therapy reported substantial energy increases in 84% and quality-of-life improvement in nearly 90%.^[26] A 10-year retrospective on subcutaneous testosterone pellets in menopausal women showed sustained reductions across all 11 Menopause Rating Scale categories.^[27]

The cardiovascular safety profile is reassuring: a 2026 systematic review and meta-analysis of 13 RCTs (n=2,628 cisgender women, 52-week transdermal testosterone) reported zero cardiovascular deaths or MACE.^[28] The dose-dependent androgenic effects (mild acne, hirsutism) are managed by titration or spironolactone.

The biggest current obstacle is supply: there is no FDA-approved female-specific testosterone formulation. Clinicians fractionate male transdermal gels off-label or use compounded products, which the global consensus statement explicitly advises against because of inconsistent dosing.^[29] A licensed female transdermal patch is the most pressing regulatory frontier.

Diagnostic and monitoring framework (Strong on protocol)

If lifestyle correction has been adequate and symptoms persist with documented biochemical deficiency, the standard protocol is:

Baseline:

Two morning total testosterone draws (peak production is shortly after waking).
Hematocrit (must be <50%); LH, FSH, prolactin, estradiol; PSA in men over 40.
Comprehensive metabolic, lipids, HbA1c.

Formulation choice (pharmacokinetics differ markedly):

Formulation	Frequency	Profile	Trade-offs
Cypionate / enanthate IM	7–14 d	Sharp peaks 36–48 h post-dose, troughs by day 14	Cheap; precise titration; highest erythrocytosis rate and mood/libido swings
Undecanoate IM (long-acting)	Initial, 4 wk, then every 10 wk	Stable steady state by day 7	Most convenient; FDA REMS — 30-min post-injection observation for rare pulmonary embolism risk
Transdermal gel (1%, 1.62%, 2%)	Daily	Approximates circadian profile	Avoids injection peaks; transfer risk to women and children on skin contact
Subcutaneous pellets	Every 3–4 mo	Steady release after mild initial spike	Excellent compliance; minor surgical procedure; hard to dose-adjust if AE occurs

Sources: ^[30], ^[31], ^[32].

Targets and follow-up:

Aim for the middle tertile of the reference range (~450–600 ng/dL), not the top — consistent with the MR finding that lifelong high-normal carries hypertensive load.
Recheck at 2–4 weeks for gels; at mid-cycle after 3–4 cycles for short-acting IM.
Stable phase: every 6–12 months on testosterone, hematocrit, PSA, lipid panel, BP.
Hematocrit >54% is the bright line: dose-reduce, lengthen interval, switch to gel, or therapeutic phlebotomy. Severe erythrocytosis raises blood viscosity and thrombotic risk.^[33]
Three-to-six-month efficacy review. If serum normalized but no symptom improvement, discuss cessation.

What's overrated

TRT as a generic longevity drug. The mortality benefit is concentrated in metabolically compromised men with documented deficiency. In healthy eugonadal men the risk-benefit is dominated by HPG-axis suppression (testicular atrophy, infertility) and erythrocytosis.
"Optimizing" healthy men into the high-normal range. The MR signal puts a real cost on lifelong elevated testosterone via blood pressure. Aim middle-tertile, not 25-year-old peak.
TRT as a libido or energy fix without OSA screening. OSA causes both, suppresses endogenous T directly, and is one of the most cost-effective things to fix in midlife — see sleep-breathing.
Compounded female testosterone products. Global consensus advises against until licensed female formulations exist.
Starting TRT without a baseline workup. The estimated 25% of patients who do this are skipping the easiest reversible causes.

Cautions

Fertility loss. Exogenous testosterone suppresses the HPG axis and halts spermatogenesis. Men who want to preserve fertility need different protocols (clomiphene, hCG, enclomiphene) — TRT is not the answer.
Polycythemia / erythrocytosis. The single most consistent adverse effect; monitor hematocrit every 6–12 months.
Atrial fibrillation, BP elevation, AKI, PE. All real signals from TRAVERSE; relevant for men with baseline risk.
Prostate. TRAVERSE found no increase in prostate cancer or worsening LUTS, debunking the older fear, but PSA monitoring remains standard of care over age 40.
Drug interactions. SSRIs, opioids, glucocorticoids, and several common medications suppress endogenous T independently — review the medication list before treating.