Testosterone (TRT)

The primary male sex hormone, prescribed as replacement (TRT) for clinically low levels. Nonmedical supraphysiologic use also occurs, and carries risks that replacement-therapy trials do not characterise. The single most-discussed compound in this space.

Testosterone binds the androgen receptor in muscle, bone, brain, skin and elsewhere, and the muscle effect comes from two things at once: more protein synthesis inside existing fibres, and satellite-cell and myonuclear changes that are observed but whose contribution to persistent "muscle memory" in humans remains uncertain. Two conversions matter as much as the parent hormone. Aromatase turns some of it into estradiol, which is not a side effect to be eliminated: men need estradiol for bone density, libido and lipids, which is why crushing it backfires. 5-alpha reductase turns some into DHT, a far more potent androgen at the hair follicle and prostate — that conversion drives most of the hair-loss and prostate conversation. Meanwhile the brain reads the blood level and throttles its own signal: exogenous testosterone suppresses LH and FSH from the pituitary, and without those two signals the testes stop producing testosterone and sperm.

For genuine hypogonadism the evidence is real but uneven by outcome: sexual function has the most consistent support, with bone density and body composition also measurable, while energy and mood effects were smaller and less consistent across trials. The TRAVERSE trial eased the cardiac fear for replacement DOSES in diagnosed men. For men with normal levels seeking enhancement, the calculus flips: supraphysiologic use builds muscle reliably (the most proven anabolic known) at the cost of everything in the risks column.

• Bhasin et al., NEJM 1996 — the study that ended the "steroids do nothing" argument. Randomised, placebo-controlled, supraphysiologic testosterone with and without training: the drug added lean mass and strength even in men who did not train at all.
• Bhasin et al., 2001 — the dose-response follow-up in healthy young men: lean mass tracked dose in a graded way, establishing that the muscle effect is a dose curve, not a switch.
• The T-Trials (NEJM 2016) — coordinated trials in men over 65 with unambiguously low levels: consistent benefit for sexual function, smaller and less certain effects on mood and walking distance, and no cognitive benefit.
• TRAVERSE (NEJM 2023) — the large cardiovascular safety trial: over 5,200 hypogonadal men aged 45-80 with existing cardiovascular disease or high risk, randomised to transdermal testosterone or placebo. Replacement-dose testosterone was non-inferior to placebo for major adverse cardiac events. No significant prostate-cancer difference emerged during its follow-up, in a screened population that excluded important prostate-risk groups — which does not establish long-term prostate safety, and endocrinology bodies still describe it as unestablished.
• The same trial found what the headlines skipped: atrial fibrillation, pulmonary embolism and acute kidney injury each occurred MORE often in the testosterone group. "Non-inferior for heart attacks and strokes" is not "no cardiovascular consequences," and clotting risk in particular deserves the weight it rarely gets. Note the scope too — replacement doses, diagnosed men, monitored. It says nothing about supraphysiologic use.
• What no trial covers: enhancement-level exposure in healthy men over years. That evidence base is observational, drawn from people who self-selected into it — useful signal, not causal proof.

Free testosterone clears from blood within minutes, so every practical form is an ester or a delivery system that slows release. Formulations differed substantially in release and elimination, and adverse effects and endocrine suppression could outlast measurable parent drug. Transdermal forms track a daily rhythm and carry a documented transfer risk to anyone in skin contact. The ester changes the delivery curve only — once cleaved, the hormone is identical in every case.

Shuts down natural production — fertility drops, testes atrophy, and recovery after years of use is slow and not guaranteed. Erythrocytosis (thick blood) requiring monitoring, lipid shifts, acne, hair loss in the predisposed, sleep apnea worsening. "TRT" obtained from a clinic that never rechecked bloodwork is enhancement wearing a lab coat. Monitoring can identify some of these abnormalities, but no monitoring scheme makes nonmedical use safe.

• Anyone trying to conceive now or in the near future — suppression of sperm production is the expected effect, not a side effect.
• Active prostate or breast cancer.
• Untreated severe sleep apnoea, until it is treated.
• Existing erythrocytosis or a personal/family history of clotting disease.
• Uncontrolled heart failure.
• Anyone with a household member who is pregnant or a young child, if using a transdermal form — contact transfer is real.

• Anticoagulants (warfarin especially) — androgens can potentiate them and raise bleeding risk; this interaction is well documented and routinely missed.
• Insulin and diabetes medication — testosterone can alter glycemic control and medication requirements; it is not a treatment for improving glycemic control.
• Untreated obstructive sleep apnoea — testosterone can worsen it, and worsened apnoea drives up haematocrit and blood pressure in turn.
• 5-alpha reductase inhibitors (finasteride) reduce DHT conversion; aromatase inhibitors reduce estradiol conversion. Both change the hormone profile substantially rather than "adding" something.
• Transdermal forms transfer by skin contact — documented virilisation in partners and children from casual contact.

Documented interactions with other things people research. A compound not listed here does not mean the combination is safe — only that no specific interaction is on file.

• Total testosterone, with free testosterone where SHBG is high or low — guidelines use the free measure when indicated rather than routinely.
• Estradiol, on a sensitive (LC-MS/MS) assay — the standard immunoassay is unreliable in men and has driven a lot of unnecessary aromatase-inhibitor use.
• Haematocrit and haemoglobin — the most common finding that actually forces a change; thickened blood raises clot risk and is the reason therapeutic phlebotomy comes up.
• Lipid panel — HDL suppression is dose-related and shows up early.
• PSA and a prostate conversation in older men — known or suspected prostate cancer remains a contraindication or a specialist-managed situation, and effects on progression are not settled.
• LH and FSH — primarily used to classify hypogonadism before treatment; a semen analysis where fertility is a concern.
• Blood pressure — the harm that compounds most quietly, and the one clinical guidelines track throughout treatment.

Stopping reveals how suppressed the axis is. The exogenous hormone clears on its ester timeline, but LH and FSH do not simply switch back on — the intervening window is a genuine low-testosterone state, with the fatigue, low mood and lost libido that implies, and it can run for months. Recovery is slower after longer duration, higher exposure, and in older men, and a minority do not fully return to their prior baseline. Persistent symptoms or fertility concerns required clinician-led endocrine evaluation; no self-directed drug strategy guaranteed recovery.

US: Schedule III anabolic steroid under the Controlled Substances Act. Clinical guidelines diagnosed hypogonadism from compatible symptoms plus consistently low morning testosterone measurements, followed by evaluation of the cause; prescribing remains subject to federal and state law.

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• Testosterone AssaysUrol Clin North Am · 2022
• Testosterone Therapy With Subcutaneous Injections: A Safe, Practical, and Reasonable OptionJ Clin Endocrinol Metab · 2022
• Testosterone Replacement OptionsUrol Clin North Am · 2022
• The impact of testosterone in men's healthEndocr J · 2023
• Testosterone physiology in resistance exercise and training: the up-stream regulatory elementsSports Med · 2010
• Testosterone and Obesity in an Aging SocietyBiomolecules · 2025
• Testosterone, testosterone therapy and prostate cancerAging Male · 2019
• Approach to the Patient: Low Testosterone Concentrations in Men With ObesityJ Clin Endocrinol Metab · 2025
• Testosterone, cardiomyopathies, and heart failure: a narrative reviewAsian J Androl · 2021
• [Testosterone - indications and abuse]MMW Fortschr Med · 2024
• Testosterone and the brainAging Male · 2006