Testosterone replacement therapy (TRT) is the medical use of exogenous testosterone to restore blood levels to the normal male range in men whose bodies no longer make enough on their own. That last clause matters: TRT is replacement, not enhancement. The goal is to bring a deficient man up to where a healthy man of his age already sits, not to push him past it. This distinction — replacement versus supraphysiologic dosing — is the single most important idea in this whole topic, and it is where most confusion between TRT and steroid use begins.
The underlying condition is male hypogonadism: a clinical syndrome of low serum testosterone together with symptoms attributable to it. It comes in two broad flavours. Primary hypogonadism is testicular failure — the testes themselves cannot produce testosterone, so the pituitary shouts louder and luteinising hormone (LH) runs high. Secondary hypogonadism is a signalling problem higher up, in the pituitary or hypothalamus, so both testosterone and LH tend to be low. Distinguishing the two changes both the workup and, sometimes, the treatment.
How it is actually diagnosed
TRT is not something a single borderline reading should trigger. Major endocrine guidelines are consistent on the diagnostic threshold, and it is stricter than most people assume:
- Two separate fasting blood tests drawn in the early morning, typically before 10 a.m., when testosterone peaks on its daily rhythm.
- Both readings below the laboratory's reference range (commonly cited around 264–300 ng/dL, roughly 9–10.4 nmol/L, at the lower bound, though ranges vary by assay).
- Symptoms that actually fit the deficiency — reduced libido, erectile difficulty, loss of morning erections, fatigue, low mood, loss of muscle and gains in fat, and in clearer cases reduced body hair or small testes.
The two-reading rule exists because testosterone is genuinely variable: it swings across the day, drops with acute illness, poor sleep, overtraining or a recent heavy meal, and can read falsely low for reasons that resolve on their own. A follow-up panel usually adds LH and FSH (to separate primary from secondary), SHBG and often free testosterone (because a normal total with high SHBG can still leave little free hormone), plus prolactin and sometimes an MRI when secondary hypogonadism is unexplained. Diagnosing off one number, or off symptoms alone, is how healthy men end up medicated for life unnecessarily.
Typical doses and targets
Delivery methods differ, but the therapeutic aim is the same: mid-normal physiological testosterone with stable levels and minimal peaks and troughs.
- Injectable esters. Testosterone enanthate and testosterone cypionate are the workhorses. Common regimens run roughly 100–200 mg weekly, and many clinicians now split the dose (for example half twice weekly) to flatten the curve. A once-every-two-weeks 200 mg shot is still prescribed but produces a much rougher ride — a supraphysiologic spike after injection and a low trough before the next.
- Long-acting depot. Testosterone undecanoate as an intramuscular depot doses only every 10–14 weeks, which is convenient but must be given in a clinical setting because of a rare risk of pulmonary oil microembolism.
- Transdermal gels and creams, applied daily, and subcutaneous pellets implanted every few months, round out the options with steadier or more hands-off profiles.
Whatever the route, treatment is titrated to a target testosterone in the middle of the normal range, guided by repeat bloodwork and — crucially — by whether symptoms actually improve. The four-ester blend sold as mixed testosterone blend Sustanon is used similarly in some countries, though its staggered esters make level-chasing slightly less predictable.
The benefits people actually get
In genuinely hypogonadal men, the human evidence is reasonably solid for several endpoints: improved libido and sexual function, better mood and energy, modest gains in lean mass and losses in fat mass, and improvements in bone mineral density over time. The 2023 TRAVERSE trial — the largest randomised cardiovascular safety trial of TRT, in middle-aged and older men with hypogonadism and cardiovascular risk — found testosterone was non-inferior to placebo for major adverse cardiac events, which reassured a long-standing safety question. It is worth being precise about the ceiling of benefit: in men whose testosterone is already normal, adding more does not reliably improve wellbeing, and the large gains people associate with 'test' come from supraphysiologic doses, which is no longer TRT.
The trade-offs — the honest part
Every man considering TRT should weigh these against the benefits, because they are real and some are not reversible:
- Fertility suppression. Exogenous testosterone signals the brain to switch off its own production, which shuts down LH and FSH and, with them, sperm production. Many men on TRT become subfertile or azoospermic. This is the biggest reason younger men who may want children are steered toward alternatives (such as hCG or clomiphene) first. hCG can be added to maintain testicular function and some fertility, but it is an extra drug, cost and injection.
- Testicular atrophy. For the same hormonal reason, the testes commonly shrink on TRT. It is usually partial and often reverses off-treatment, but it bothers many men.
- Elevated hematocrit. Testosterone stimulates red-blood-cell production, and a rising hematocrit (polycythaemia) is the most common measurable side effect. Left unchecked it thickens the blood and theoretically raises clot risk, so hematocrit is monitored and managed with dose reduction or, occasionally, blood donation.
- Estrogen and its symptoms. Some testosterone aromatises to estradiol; a minority of men develop breast tenderness or gynaecomastia. Aromatase inhibitors such as anastrozole (or exemestane, or letrozole) are sometimes used, but crashing estradiol causes its own harms to libido, joints, mood and lipids, so they are used sparingly and to a target, not reflexively.
- Other monitored effects. Acne and oily skin, fluid retention, possible worsening of untreated sleep apnoea, and lipid shifts. Prostate cancer is not caused by restoring normal testosterone on current evidence, but pre-existing prostate cancer is a contraindication, and PSA is tracked.
- Lifelong dependence. This is the trade-off people underrate most. Because TRT suppresses the natural axis, stopping it — especially after years — can leave a man temporarily worse off than before, until his own production recovers, if it fully does. For primary hypogonadism the axis was never going to recover anyway; for many others, TRT is effectively a permanent commitment. Going on is a decision to keep going on.
Bottom line
TRT is a legitimate, well-studied treatment that reliably helps men who are actually deficient, diagnosed properly on two low morning readings plus symptoms, and dosed to restore a normal — not supraphysiologic — level. The benefits to libido, mood, body composition and bone are real. So are the costs: suppressed fertility, testicular shrinkage, a hematocrit you have to watch, and, for most men, a lifelong dependence on the therapy. It is a good deal for the genuinely hypogonadal and a poor one for men chasing an edge they do not medically need. None of this is medical advice; it is reference information to inform a conversation with a clinician.