Testosterone Replacement Therapy (TRT): A Complete, Evidence-Based Guide

Few areas of men's health are surrounded by as much hype, fear, and confusion as testosterone replacement therapy. This guide cuts through it with what the evidence actually shows in 2026 — including the largest safety trial ever run and the FDA decision that followed it.

Testosterone replacement therapy (TRT) is a medical treatment that restores testosterone to a healthy range in men whose bodies no longer produce enough. Done well, it can relieve real symptoms. Done casually — without a proper diagnosis or monitoring — it carries avoidable risks. The difference comes down to evidence, testing, and follow-up.

Below, we walk through what TRT is, who is actually a candidate, what the science says about its benefits and risks, and how to think about treatment safely. None of this is medical advice; decisions about TRT should always be made with a qualified clinician.

The short version

• TRT treats diagnosed testosterone deficiency (hypogonadism) — a combination of consistently low blood levels and symptoms. It is not a general anti-aging tool.

• Diagnosis requires two low morning blood tests, not a single reading or symptoms alone.

• The 2023 TRAVERSE trial — the largest TRT safety study to date — found no increase in major cardiovascular events in men with hypogonadism and high cardiac risk.

• In February 2025, the FDA removed the cardiovascular boxed warning from all testosterone products, while adding a new warning about increased blood pressure.

• TRT suppresses fertility. Men who want children should discuss alternatives first.

• Safe therapy depends on ongoing monitoring of testosterone, hematocrit (red blood cell concentration), and PSA.

What testosterone does — and what "low T" really means

Testosterone is the primary male sex hormone. Beyond reproduction and libido, it influences muscle mass, bone density, red blood cell production, mood, energy, and cognition. Levels naturally decline with age — roughly 1% per year after 30 — but a gradual dip is not the same as a clinical deficiency.

Hypogonadism is the medical term for a true deficiency. It comes in two forms: primary (the testes can't produce enough testosterone) and secondary (the brain's signaling to the testes is impaired). Identifying which one a man has changes the workup and the treatment plan, which is why self-diagnosis is unreliable.

How testosterone deficiency is diagnosed

A diagnosis requires both low blood levels and consistent symptoms. Major medical societies set the biochemical thresholds slightly differently:

• Endocrine Society: low total testosterone below ~264 ng/dL

• American Urological Association (AUA): below 300 ng/dL

• European and international sexual-medicine bodies (EAU, ISSM, ISSAM): below ~350 ng/dL

Because testosterone peaks in the morning and fluctuates day to day, guidelines call for at least two separate morning measurements before making a diagnosis. Clinicians often add free testosterone, LH, FSH, prolactin, SHBG, and estradiol to understand the underlying cause.

Common symptoms that, combined with low labs, point toward deficiency include low libido, erectile difficulty, persistent fatigue, loss of muscle, increased body fat, depressed mood, and reduced concentration. Crucially, these symptoms are nonspecific — they overlap with thyroid problems, sleep apnea, depression, anemia, and more. That overlap is exactly why testing matters.

What the science says about benefits

In men with genuine hypogonadism, the evidence for benefit is reasonably strong for several outcomes:

• Sexual function: improved libido and, in many men, erectile function.

• Body composition: increased lean muscle mass and reduced fat mass.

• Bone density: measurable improvements over time.

• Anemia: TRAVERSE confirmed testosterone corrects anemia in deficient men.

• Mood and energy: many men report improvement, though results are more variable and harder to measure.

It's worth being precise about the line between evidence and marketing. TRT reliably helps men who are actually deficient. There is little good evidence that pushing testosterone into supraphysiologic ranges in men with normal levels produces added benefit — and doing so adds risk.

What the science says about risks

For years, TRT carried an FDA boxed warning about possible cardiovascular harm, based on weak and conflicting data. The TRAVERSE trial (published 2023) was designed to settle the question. It randomized 5,246 men aged 45–80 with hypogonadism and existing or high cardiovascular risk to testosterone gel or placebo.

After an average of about 33 months, major adverse cardiac events (heart attack, stroke, or cardiovascular death) occurred in 7.0% of the testosterone group versus 7.3% of placebo — meeting the bar for non-inferiority. In other words, in this population, testosterone did not increase major cardiac risk.

On the strength of that data, the FDA removed the cardiovascular boxed warning from all testosterone products in February 2025, replacing it with a warning about increased blood pressure.

That reassurance comes with important caveats. TRAVERSE did flag some secondary signals that warrant attention and monitoring:

• A higher rate of non-fatal cardiac arrhythmias (notably atrial fibrillation).

• More venous blood clots (pulmonary embolism / deep vein thrombosis).

• More fractures in the testosterone group — an unexpected finding still being studied.

• A measurable rise in blood pressure, now reflected on product labels.

Separately, the most common side effect in routine practice is erythrocytosis — testosterone thickening the blood by raising red blood cell count. This is dose-dependent and manageable: clinicians typically adjust the dose if hematocrit exceeds ~52% and pause therapy above 54%.

On the prostate: TRT does not appear to cause prostate cancer, but it can raise PSA, so PSA is checked before starting and monitored during treatment. Therapy is generally not started in men with an unexplained elevated PSA without urological evaluation.

TRT and fertility — read this before starting

This is the single most under-appreciated fact about TRT: it suppresses the body's own testosterone production and sperm production. For reproductive-age men who want to father children, starting TRT can cause temporary — sometimes prolonged — infertility.

The good news is there are alternatives that raise testosterone while preserving fertility, including clomiphene, enclomiphene, and hCG. Any man who may want children should raise fertility before beginning treatment, not after.

Common misconceptions

• "TRT is just an anti-aging supplement." It's a prescription treatment for a diagnosed medical condition, not a wellness add-on.

• "Low energy means I need testosterone." Fatigue has many causes. Low testosterone is one possibility among many, and only testing can tell.

• "TRT causes prostate cancer." Current evidence does not support this, though monitoring is still required.

• "More testosterone is better." The goal is restoring a healthy range, not maximizing a number. Higher doses mainly add side effects.

• "It's a quick fix." TRT is typically a long-term commitment that requires ongoing bloodwork and follow-up.

Treatment options

Testosterone can be delivered several ways, each with trade-offs:

• Injections (intramuscular or subcutaneous) — the most common method (~70% of patients), inexpensive and effective, but levels can peak and trough between doses and they raise hematocrit more than other forms.

• Gels and creams (~20%) — applied daily for steadier levels, but more expensive and carry a risk of skin-to-skin transfer to partners or children.

• Pellets — implanted under the skin every 3–6 months for convenience, but the dose can't be adjusted once placed.

• Patches, oral, and nasal formulations — additional options that suit specific situations.

The "best" option depends on a man's biology, lifestyle, fertility goals, and how his levels respond — which is why this is a conversation to have with a knowledgeable provider.

Monitoring: the part that makes TRT safe

Responsible TRT is defined less by the prescription and more by the follow-up. Standard monitoring includes testosterone levels to confirm an appropriate range, hematocrit to catch blood thickening, and PSA (typically at 3–6 months and 12 months, then periodically). Blood pressure should also be tracked given the updated label. Men who aren't willing to commit to regular bloodwork are not good candidates for therapy.

When to seek medical care

Talk to a clinician if you have persistent symptoms of low testosterone — low libido, erectile difficulty, unexplained fatigue, loss of muscle, or low mood — especially alongside risk factors like obesity, type 2 diabetes, or a history of testicular problems. Seek prompt care if, while on TRT, you develop chest pain, shortness of breath, leg swelling or pain (possible clot), or palpitations.

Frequently asked questions

Is TRT safe for the heart?
In men with diagnosed hypogonadism, the TRAVERSE trial found no increase in major cardiac events, and the FDA removed its cardiovascular boxed warning in 2025. Monitoring for blood pressure, blood clots, and arrhythmia is still recommended.

Will TRT make me infertile?
It can. TRT suppresses sperm production. If fertility matters to you, discuss alternatives like clomiphene, enclomiphene, or hCG before starting.

How long until TRT works?
Libido and energy may improve within weeks; changes in muscle, fat, and mood typically unfold over 3–6 months or longer.

Do I have to stay on TRT for life?
Often, yes. Stopping usually returns testosterone to its previous low level, and the body's own production may take time to recover.

What testosterone level is "low"?
Generally below 264–300 ng/dL depending on the guideline, confirmed on two morning tests and paired with symptoms.

Can I just buy testosterone online?
Unsupervised testosterone — without diagnosis or monitoring — is where most of the avoidable harm happens. A proper workup protects you.

Key takeaways

• TRT is a treatment for diagnosed testosterone deficiency, not a general anti-aging or performance tool.

• Diagnosis requires two low morning blood tests plus symptoms.

• The TRAVERSE trial and the 2025 FDA label change substantially eased long-standing cardiovascular fears in appropriately selected men.

• Real risks remain — blood thickening, blood pressure, clots, and fertility suppression — all manageable with proper monitoring.

• The safest TRT is supervised TRT, with regular bloodwork and a provider who knows your full picture.

Find the right provider for your situation

If you're wondering whether your symptoms point to low testosterone, the next step is a proper evaluation — not a guess. Sidewalk helps you complete a free, AI-guided health assessment and connect with trusted local providers who specialize in hormone optimization and men's health, with your goals and any lab data ready before your first visit.

Complete a free AI health assessment or find a provider who specializes in testosterone and men's health.

This article is for educational purposes only and is not a substitute for professional medical advice. Always discuss treatment decisions, including TRT, with a qualified healthcare professional.