What is TRT (Testosterone Replacement Therapy)? | PeptideWorld

What is TRT (Testosterone Replacement Therapy)?

🧬 Hormone Health ⏱ 13 min read 🎓 Beginner – Intermediate
Medical Disclaimer: This article is for educational purposes only and does not constitute medical advice. TRT is a prescription medication that should only be initiated under the supervision of a licensed physician following proper diagnostic evaluation.

Testosterone replacement therapy has undergone a significant rehabilitation in the medical literature over the past three years. Long overshadowed by cardiovascular safety concerns that turned out to be less substantiated than initially feared, TRT now has the backing of a major randomised controlled trial, a 2025 FDA labelling update removing a decade-old warning, and clearer clinical guidelines than at any point in its history.

This guide explains what TRT actually is, who it is and is not appropriate for, what the best current evidence shows about its benefits and remaining risks, and what the decision process looks like when working with a physician.

Key Takeaways

  • The 2023 TRAVERSE trial (5,246 men, published in NEJM) found TRT is non-inferior to placebo for major adverse cardiac events in men with documented hypogonadism — the most significant cardiovascular safety data for TRT ever produced.
  • In February 2025, the FDA removed the black box warning about cardiovascular risk from testosterone product labelling, based on the TRAVERSE findings.
  • TRT is appropriate for men with documented hypogonadism — two morning testosterone levels below 300 ng/dL plus clinical symptoms. It is not appropriate for normal age-related testosterone decline in the absence of symptoms.
  • Benefits are well-evidenced for libido, body composition, mood, bone density, and anaemia correction. Sexual function benefits depend heavily on the underlying cause of dysfunction.
  • TRT suppresses spermatogenesis in all users — it is effectively a contraceptive and fertility implications must be discussed before starting in any reproductively-active man.
  • Key risks that remain after TRAVERSE: erythrocytosis (elevated red blood cell count), atrial fibrillation (modestly elevated), and testicular atrophy. Prostate cancer risk appears not significantly elevated in appropriately monitored patients.

The Most Important Recent Development: The TRAVERSE Trial

For nearly a decade, testosterone therapy was shadowed by a 2010 trial that was halted early after suggesting increased cardiovascular events — a finding that prompted a 2015 FDA black box warning and dramatically reduced TRT prescribing rates. The TRAVERSE trial, published in the New England Journal of Medicine in 2023, provided the first large-scale, definitive answer to the cardiovascular safety question.[1]

The TRAVERSE Trial — Key Facts

5,246
Men enrolled (aged 45–80)
27.1
Mean months of follow-up
7.0%
MACE rate — TRT group
(vs 7.3% placebo)

Men with documented hypogonadism (two morning testosterone levels below 300 ng/dL) and pre-existing or high cardiovascular risk were randomised to daily transdermal testosterone gel or placebo. The primary endpoint — major adverse cardiac events (death, non-fatal heart attack, non-fatal stroke) — showed TRT was non-inferior to placebo. There were 16 fewer deaths in the TRT group (non-significant). No significant increase in prostate cancer was observed. The trial was the largest RCT of testosterone therapy ever conducted.

📋 February 2025: FDA Removes Black Box Cardiovascular Warning Following the TRAVERSE results, the FDA updated the labelling for all testosterone products in February 2025, removing the black box warning related to increased cardiovascular risk. The FDA maintained existing restrictions around age-related use (TRT remains formally approved only for hypogonadism from specific medical causes, not age-related decline), but the decade-long shadow over TRT's cardiovascular safety profile was formally lifted based on the evidence.

What TRT Actually Is

Testosterone replacement therapy provides exogenous testosterone to men whose bodies are not producing sufficient amounts — restoring levels to within the normal physiological range. It does not produce supraphysiological levels (which is the domain of anabolic steroid misuse) — well-managed TRT targets the same range that healthy young adult men maintain naturally.

The mechanism is straightforward: exogenous testosterone is absorbed into the bloodstream, where it circulates as either free testosterone (biologically active) or bound to proteins including sex hormone binding globulin (SHBG) and albumin. Free testosterone enters cells and binds to androgen receptors, triggering the downstream effects — protein synthesis, gene expression changes, and the full spectrum of testosterone's biological functions.

Who TRT Is — and Is Not — Appropriate For

✓ TRT is clinically appropriate for men who have:

  • Two morning testosterone levels below 300 ng/dL on separate occasions
  • Clinical symptoms of hypogonadism (libido, energy, body composition, mood)
  • Symptoms not explained by other conditions (thyroid, sleep apnoea, obesity, medications)
  • A clear cause of hypogonadism (primary: testicular failure; secondary: pituitary/hypothalamic)
  • Willingness to commit to ongoing monitoring and medical supervision

✗ TRT is NOT appropriate for men who:

  • Have normal testosterone levels but attribute non-specific symptoms to "low T"
  • Have testosterone in the low-normal range without clinical symptoms
  • Have not addressed reversible causes (obesity, sleep apnoea, chronic stress)
  • Want to build muscle above natural capacity (anabolic steroid use, not TRT)
  • Have active prostate cancer or prostate cancer history without specialist clearance
  • Have a haematocrit above 54% or uncontrolled polycythaemia
  • Want to preserve fertility without concurrent fertility preservation measures

The Age-Related Testosterone Decline Question

Men's testosterone declines approximately 1% per year after age 40 — a real biological change. The FDA currently approves TRT only for hypogonadism from specific medical causes, not age-related decline. However, the American Urological Association and Endocrine Society guidelines recommend considering TRT in men with confirmed low testosterone levels and clinical symptoms, regardless of cause. Many physicians operate in this space. The critical distinction is: symptoms present, levels confirmed low, reversible causes addressed. "I'm in my 50s and feel tired" is not sufficient without proper diagnostic workup.

What TRT Evidence Shows for Benefits

Strong Evidence Sexual Function & Libido Improved libido is among the most consistently documented benefits. The T Trials (a multi-site US study) found sexual activity increased in the TRT group vs placebo. However, erectile function improvement depends heavily on the underlying cause — TRT helps when the cause is hormonal; it is less effective for erectile dysfunction driven by vascular or neurological factors. Men with severe diabetes may see less sexual function benefit.
Strong Evidence Body Composition TRT consistently increases lean muscle mass and reduces fat mass, particularly visceral fat. These effects are dose-dependent and enhanced by concurrent resistance training. The body composition benefits are among the most reliably produced effects of TRT, with measurable changes typically visible within 3–6 months.
Strong Evidence Bone Density Testosterone is critical for bone mineral density in men. Hypogonadism is a significant risk factor for osteoporosis and fracture. TRT reliably improves bone density over 12–24 months. Bone density is monitored as part of long-term TRT management.
Strong Evidence Anaemia Correction TRAVERSE confirmed that TRT effectively corrects mild anaemia in hypogonadal men — an often overlooked benefit. Testosterone stimulates erythropoietin production and red blood cell production, which can address unexplained normocytic anaemia in older men with low testosterone.
Moderate Evidence Mood, Energy & Motivation Improved energy, motivation, and mood are consistently reported in clinical practice and in trial data. The degree of improvement depends on baseline levels and symptom severity. Men with the lowest testosterone and most pronounced symptoms typically report the most benefit. Subclinical depression associated with hypogonadism often responds well.
Moderate Evidence Metabolic Health TRT improves insulin sensitivity and may reduce progression from prediabetes to type 2 diabetes in hypogonadal men. The TRAVERSE diabetes substudy showed reduced need for diabetes medication in some men receiving TRT, though the primary HbA1c endpoint did not differ significantly. The metabolic benefits are real but variable.

Delivery Methods

Daily
Transdermal Gel Applied daily to upper arms, shoulders, or inner thighs. Most studied delivery method (used in TRAVERSE). Produces stable testosterone levels. Risk of transference to partners or children via skin contact — must be managed. Most common first-line option.
Weekly / Biweekly
Subcutaneous or Intramuscular Injection Testosterone cypionate or enanthate. Injections produce a peak-and-trough testosterone pattern. Subcutaneous (small needle, belly or thigh fat) is increasingly preferred over intramuscular for self-administration. Lower cost than gels; more stable with subcutaneous weekly dosing.
Daily
Oral Testosterone Undecanoate Jatenzo and Tlando — newer oral formulations that avoid the liver toxicity of older testosterone pills by absorbing through the lymphatic system. Convenient. Must be taken with food containing fat. Increasing in popularity since approval in 2019–2022.
Every 3–6 months
Subcutaneous Pellets Pellets inserted under the skin (typically buttock area) by a physician. Release testosterone slowly over months. Highest convenience — no daily or weekly dosing. Limited ability to adjust dose between insertions. Popular in some functional medicine settings.
Daily (3x/day)
Intranasal (Natesto) Applied inside the nostril three times daily. Less testicular suppression than other routes — potentially relevant for men wanting to preserve some fertility. Lower overall testosterone levels than other methods. Less commonly used.
Every 10 weeks
Long-Acting Injectable (Aveed) Testosterone undecanoate injected intramuscularly by a physician in a clinical setting every 10 weeks after initial loading doses. Very stable levels. Requires clinical administration due to risk of oil embolism — cannot self-administer.

Risks That Remain After TRAVERSE

Risk Evidence Status Clinical Significance & Management
Erythrocytosis
(elevated haematocrit)		 Well-established — most common adverse effect TRT stimulates red blood cell production. Haematocrit above ~54% raises blood viscosity and venous thromboembolism risk. Managed with dose reduction or therapeutic phlebotomy (blood donation). Haematocrit must be monitored regularly.
Atrial Fibrillation Modestly elevated in TRAVERSE (not associated with increased MACE) TRAVERSE found a small increase in atrial fibrillation incidence in the TRT group. This was not accompanied by increased heart attack or stroke. Requires discussion in men with pre-existing arrhythmia risk.
Testicular Atrophy Universal — expected effect Exogenous testosterone suppresses LH and FSH, reducing the pituitary signal to the testes. Testicular size reduces with time on TRT. This is a cosmetic rather than functional concern in most men, but is significant for fertility (see below).
Blood Pressure Small elevation noted in TRAVERSE A modest increase in blood pressure was observed. Men with pre-existing hypertension require closer monitoring. Blood pressure should be assessed at each follow-up.
Prostate Cancer Evidence suggests risk not significantly elevated in appropriately monitored patients TRAVERSE found no significant difference in prostate cancer rates (5 high-grade vs 3 placebo — non-significant). Current expert consensus is that TRT does not cause prostate cancer, though PSA monitoring is mandatory. TRT remains contraindicated in active prostate cancer.
Skin reactions (gel users) Common, mild Local skin irritation at application site. Transference to partners or children if skin-to-skin contact before gel dries — requires patient education on application protocols.

The Fertility Question — A Critical Conversation

⚠️ TRT Is Effectively a Contraceptive — This Must Be Discussed Before Starting Testosterone replacement therapy suppresses the hypothalamic-pituitary-gonadal (HPG) axis. Exogenous testosterone signals the brain that sufficient testosterone exists, causing the pituitary to reduce or stop LH and FSH secretion. Without LH and FSH, the testes stop producing their own testosterone — and stop producing sperm. This effect is reliable enough that high-dose testosterone has been studied as a male contraceptive. Spermatogenesis typically returns after stopping TRT, but recovery can take months to over a year, and is not guaranteed in all men. For any man who may want biological children in the future, alternatives that preserve fertility should be discussed first: clomiphene citrate, enclomiphene, or hCG stimulate endogenous testosterone production through the pituitary rather than bypassing it. These are covered in the TRT vs Natural Testosterone Optimisation article in this hub.

Monitoring While on TRT

Standard TRT Monitoring Protocol

Testosterone level
Checked at 3 months then annually. Target is typically 400–700 ng/dL for total testosterone. Free testosterone and SHBG should also be tracked. Testing should be done mid-cycle for injections (not at peak or trough).
Haematocrit
Checked at 3 months, 6 months, then annually. The most important safety marker. If haematocrit exceeds 54%, dose should be reduced or phlebotomy performed. Some men are more susceptible than others.
PSA
Prostate-specific antigen checked at baseline, 3–6 months, then annually. A significant rise (>1.4 ng/mL above baseline within 12 months) warrants urological evaluation. Not an absolute contraindication to TRT, but requires investigation.
Blood pressure
Monitored at follow-up visits given the modest blood pressure elevation observed in TRAVERSE. Particularly important in men with pre-existing hypertension or cardiovascular risk.
Estradiol
Some testosterone converts to estrogen via aromatase. Elevated estradiol in men causes breast tenderness, mood changes, and water retention. Checked if symptoms suggest, and routinely in men on higher doses or with higher body fat.
Symptoms review
The clinical goal is symptomatic improvement, not just a lab number. If symptoms have not improved at adequate testosterone levels, TRT may not be the primary driver of the complaint — the diagnostic work-up should continue.

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References

  1. Lincoff AM, et al. Cardiovascular Safety of Testosterone-Replacement Therapy. N Engl J Med. 2023;389(2):107–117. Available from: https://pubmed.ncbi.nlm.nih.gov/37326322/
  2. Walia A, et al. Testosterone Replacement, Where Are We in 2025? Trends Urol Mens Health. 2025. Available from: https://onlinelibrary.wiley.com/doi/10.1002/tre.70016
  3. Zitzmann M, et al. Cardiovascular safety of testosterone therapy — Insights from the TRAVERSE trial and beyond. Andrology. 2026. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC12670475/
  4. Cleveland Clinic. TRAVERSE Study Supports Cardiovascular Safety of Testosterone Therapy When Used as Indicated. 2023. Available from: https://consultqd.clevelandclinic.org
  5. Healthline. Testosterone Replacement Therapy: FDA Panel Calls for Expanded Access. December 2025. Available from: https://www.healthline.com
  6. Grand Rounds in Urology. Testosterone and Cardiovascular Risk: TRAVERSE Trial and New FDA Label Change. February 2025. Available from: https://grandroundsinurology.com
  7. Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. Available from: https://pubmed.ncbi.nlm.nih.gov/29562364/