Testosterone replacement therapy, commonly abbreviated TRT, is medical treatment to raise testosterone levels through external administration, and for longevity purposes it is genuinely important to distinguish two different use cases with two considerably different evidence bases, treating clinically diagnosed hypogonadism, well-established, versus using it for general anti-aging purposes in men with already-normal testosterone, meaningfully less evidenced.
- Well-established for treating clinically diagnosed hypogonadism specifically.
- Less evidenced for general anti-aging use in men with normal testosterone.
- Known risks include effects on red blood cell count and fertility suppression.
- Requires medical diagnosis and supervision, not a decision to make independently. DHEA, an adrenal precursor sold over the counter, declines on a similar curve with a considerably weaker evidence base.
How low testosterone is diagnosed and monitored
How hypogonadism is diagnosed
Diagnosis requires both consistent symptoms and repeatedly low measured testosterone. The measurement side is more demanding than most people expect:
- Morning sampling. Levels follow a daily rhythm and peak in the early morning. An afternoon sample can read low in a man with entirely normal production.
- At least two separate measurements. Day-to-day variation is substantial, and acute illness suppresses levels.
- Total and free testosterone. Sex hormone binding globulin rises with age, so total testosterone can look adequate while the free fraction is low. Obesity does the reverse.
- LH and FSH. These distinguish primary testicular failure from a pituitary or hypothalamic cause, which changes management entirely.
- Morning sampling. Testosterone follows a daily rhythm, peaking in the morning. A guideline-concordant test is taken before roughly 10am, and an afternoon sample can read substantially lower in the same man.
- Two separate measurements. Single readings vary enough that at least two low results on different days are standard before diagnosis.
- Total and free testosterone. Most circulating testosterone is bound to sex hormone binding globulin. When SHBG is abnormal, which is common with obesity, thyroid disease or aging, total testosterone misrepresents what is biologically available, and free or calculated free testosterone is more informative.
- LH and FSH. These distinguish primary testicular failure from a pituitary or hypothalamic cause, which changes both the diagnosis and the appropriate treatment.
Thresholds vary by laboratory and assay, which is a genuine problem in this field rather than a footnote. Values around 300 ng/dL are commonly used as a lower boundary, with symptoms required alongside the number.
What reversible causes to exclude first
Obesity, obstructive sleep apnea, excessive alcohol, opioid use, overtraining and severe energy restriction all suppress testosterone, and all are correctable. Treating the suppression with exogenous testosterone leaves the cause in place and makes the suppression permanent, since supplied testosterone shuts down the body's own production.
What treatment does and doesn't deliver
In diagnosed hypogonadism, treatment reliably improves sexual function, bone density, lean mass and often mood. In men with age-related decline but levels inside the reference range, the effects on energy, mood and vitality are considerably smaller and less consistent than clinic marketing implies. Body composition changes are real in both groups; how much anyone notices depends on the starting point.
Monitoring once treatment starts
The cardiovascular question that dominated this field for a decade was addressed by the TRAVERSE trial, reported by Lincoff and colleagues in 2023: testosterone was noninferior to placebo for major adverse cardiac events in men with hypogonadism and elevated cardiovascular risk. That does not make the treatment consequence-free, since the same trial found higher rates of atrial fibrillation, acute kidney injury and pulmonary embolism.
- Haematocrit. Testosterone stimulates red cell production, and rising haematocrit is the most common reason to reduce or stop. This is the single strongest argument against unsupervised use.
- PSA. Monitored on the basis that treatment doesn’t appear to cause prostate cancer but may accelerate an existing one.
- Testosterone level itself. To confirm the dose reaches the target range without overshooting.
Haematocrit is the most important routine check, since testosterone stimulates red blood cell production and excessive thickening of the blood carries thrombotic risk. PSA is monitored in older men. Fertility should be discussed before starting rather than after, because suppression of sperm production is expected and recovery is neither guaranteed nor quick.
None of this makes TRT inappropriate for genuine hypogonadism, where the evidence for symptom improvement is solid. It makes the difference between a diagnosis and a prescription written off symptoms alone.
Exogenous testosterone suppresses the pituitary signalling that drives sperm production, frequently to infertility. Recovery after stopping is usual but neither guaranteed nor rapid, and can take many months. For anyone who may want children, this belongs at the front of the conversation rather than in a footnote, and alternatives that preserve the signalling axis exist.
The well-established use case
Clinically diagnosed hypogonadism, confirmed low testosterone through blood testing combined with corresponding symptoms, low energy, reduced muscle mass, low libido, mood changes, is the well-established, robustly evidence-backed indication for TRT, with genuine, well-documented benefits for these specific symptoms in this diagnosed population. This is standard, well-accepted medical practice with decades of clinical experience and research behind it.
The thinner-evidenced anti-aging use case
A different, growing use case involves men with already-normal testosterone levels using TRT specifically to push those levels higher, seeking general anti-aging, performance, or wellness benefits rather than treating a diagnosed deficiency. This use case rests on a thinner evidence base, the well-established research on TRT largely comes from studies of clinically hypogonadal populations, and the benefits and risk profile don't necessarily generalize cleanly to healthy men starting from a normal baseline, worth being honest about this evidence gap rather than assuming the same strong evidence applies equally to both situations.
Almost all confusion about TRT comes from collapsing two different situations into one word. Diagnosed hypogonadism and age-related decline have different evidence bases, different risk profiles and different decisions attached.
The diagnostic process matters here because the symptoms attributed to low testosterone — fatigue, low mood, reduced libido, loss of muscle — are non-specific and have many other causes, several more common than hypogonadism.
Two use cases, side by side
| Diagnosed Hypogonadism | General Anti-Aging Use | |
|---|---|---|
| Starting testosterone level | Confirmed clinically low | Already normal |
| Evidence base | Well-established, decades of research | Considerably thinner |
| Standard medical practice? | Yes | Off-label, growing but less established |
Two use cases sharing a name and a drug, with different evidence bases behind them.
Two treatments that share a name
Consumer guidance blurs the two use cases together, implying the strong evidence for diagnosed hypogonadism applies equally to general anti-aging use.
Replacement for a diagnosed deficiency has real evidence and established clinical use. Raising a level that is already in range is a different treatment with a much thinner evidence base and a different risk picture.
Which one you are considering changes everything about how to read the research. That distinction is worth being clear on before a consultation, not after.
The app is honest about which interventions have strong evidence, and which don't yet.
Sources
Key references for the claims on this page. Where a figure is attributed to a specific study or body, it is named here.
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. 2018;103(5):1715–1744. DOI The diagnostic and monitoring standards described here.
- Lincoff AM, et al. Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE). New England Journal of Medicine, 2023.
Frequently asked
What is TRT well-established for treating?
Clinically diagnosed hypogonadism, confirmed low testosterone with corresponding symptoms.
Is it well-supported for general anti-aging use?
Less evidenced, the strong evidence base is largely from diagnosed hypogonadal populations.
What are the known risks?
Effects on red blood cell count, fertility suppression, and cardiovascular considerations under active research.
What are the side effects of TRT?
Acne, elevated red blood cell count, testicular shrinkage and suppressed fertility are the common ones. Sleep apnea can worsen. Red cell count in particular needs monitoring, which is one reason unsupervised use is a poor idea.
Does TRT cause infertility?
It suppresses the body's own signalling to produce sperm, and this is one of the more consistent effects. Fertility often recovers after stopping, but not always and not quickly, which matters if children are still a possibility.
Is TRT a lifelong commitment?
For diagnosed hypogonadism it is usually ongoing, because stopping returns you to the state that prompted treatment. Natural production may take months to recover afterwards and sometimes doesn’t fully return.
What testosterone level counts as low?
Thresholds vary between laboratories, and levels swing across the day, so a morning sample and at least one repeat test are standard before anything is concluded. A single afternoon reading isn’t a diagnosis.
Can you raise testosterone naturally?
Improving sleep, losing excess fat, training with resistance and cutting heavy alcohol use all help modestly, and are worth doing regardless. None of them will correct genuine clinical hypogonadism, which is a different problem.