−1% / yr
Population-level testosterone drift, independent of age alone
Travison 2007 · MMAS
+32%
Insulin-stimulated glucose uptake after six months
Dhindsa 2016 · Diabetes Care
Bone
Volumetric density and estimated strength rose in older men
Testosterone Trials bone substudy
Educational content, not medical advice. Testing and treatment decisions belong with a physician.
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What the trials actually measured
Many claims about testosterone treatment rest on anecdote and expectation. Here we stick to what controlled trials and longer follow-ups have actually measured in men with confirmed deficiency. That is an important boundary: the results apply to men in whom both symptoms and repeated blood tests point to low testosterone, not to healthy men who want more.
Bhasin and colleagues studied in controlled trials how different doses of testosterone affect the body. Fat-free mass and muscle strength tracked the level in the blood: the higher the exposure, the larger the change. That is body composition measured with objective methods, not a mood score. The trials also show that the effect depends on dose, which is why the dose must be managed by a doctor using measurements.
Muscle mass matters more than appearance. With age many men lose muscle and strength, which is linked to fall risk, poorer metabolism and loss of independence. That a treatment can measurably shift fat-free mass in deficient men is therefore relevant to health, not only to training.
Glucose and insulin sensitivity
Dhindsa and colleagues (Diabetes Care, 2016) studied men with type 2 diabetes and low testosterone. They measured insulin sensitivity with the clamp method, the most precise method available, not with a questionnaire. After six months of treatment, insulin-stimulated glucose uptake rose by about 32 percent compared with placebo. At the same time fat mass fell and fat-free mass rose.
Haider and colleagues followed men with both type 2 diabetes and low testosterone in a registry for up to eleven years. Some of the treated men achieved remission of their diabetes, which was not seen in the comparison group. That is an important finding, but it is a registry study and not a randomised trial. It shows an association, not proof of cause, and should be read that way.
Bone
The Testosterone Trials were a set of coordinated, placebo-controlled trials in older men with low testosterone. In the bone substudy (Snyder and colleagues, JCEM), volumetric bone density and estimated bone strength rose in the treatment group, especially in the spine. The trial was not large enough to show fewer fractures, so one cannot say that treatment prevents fractures. But it shows that bone responds measurably.
Sexual function and mood
In the same series of trials (Snyder and colleagues, NEJM, 2016), treatment moderately improved sexual desire, sexual activity and erectile function compared with placebo. The effect on mood and depressive symptoms was small, and the effect on energy and walking distance was modest. That is an honest picture: treatment helps in some areas, less in others, and it is not a universal fix.
Heart and safety
The TRAVERSE trial (NEJM, 2023) studied men with low testosterone and increased cardiovascular risk. Treatment did not increase the rate of major cardiovascular events compared with placebo. On the other hand, slightly more cases of atrial fibrillation, acute kidney injury and pulmonary embolism were seen in the treatment group. That is exactly why treatment requires follow-up.
Testosterone also increases red blood cell production, so haematocrit must be measured before and during treatment. Added testosterone also suppresses the body's own signals to the testicles and can reduce sperm production. Men who want children should discuss this with the doctor before starting.
The population and "normal"
Travison and colleagues (JCEM, 2007) showed in the MMAS study that population testosterone levels fell by about one percent per year between 1987 and 2004, independent of age alone. Ageing and a falling population baseline are not the same claim. A result printed as normal says something about the men around you today, not necessarily about what fits your symptoms.
Who may benefit
The benefits in the trials were seen in men with confirmed deficiency: symptoms that fit, and repeated morning samples with low total or low free testosterone. In men with normal values there is no evidence of the same effect. Other causes, such as excess weight, sleep apnoea, medication and pituitary disease, must have been assessed. Many men also have downstream conditions such as type 2 diabetes, abdominal fat or reduced bone density, and here it can be relevant to look at testosterone as part of the whole picture.
What it does not mean
None of this is a reason to skip measurement, haematocrit monitoring or a licensed doctor. Nor is it a promise that treatment will work for everyone. It is a reason not to treat low energy, belly fat and a "normal" lab result as three separate stories that are never seen together.
Next step
Write down your symptoms. Take a morning sample with total testosterone, SHBG and calculated free testosterone. The first test can be taken locally, where you live. A doctor then assesses the numbers together with your symptoms, your medication and your history, and any decision about treatment belongs to the doctor.
Clinical context
Total Testosterone vs. Symptom Ranges
Hover or focus a point for age, level and both unit systems.
Early / individual symptoms13–19 nmol/L · 375–548 ng/dLZitzmann et al. (2006) found the first symptom associations around 15 nmol/L: reduced libido, declining vigor and energy, weaker morning erections, poorer recovery after exercise, lower motivation and flatter mood. Symptoms are individual and relate to the fall from your own younger baseline — not a fixed cut-off.
Sexual symptoms become more likely11–13 nmol/L · 317–375 ng/dLBelow ~12–13 nmol/L the associations widen in Zitzmann et al. (2006) and EMAS: markedly reduced sex drive, erectile dysfunction, absent morning erections, reduced physical strength and endurance, growing belly fat, difficulty building or maintaining muscle, and increasing insulin resistance / prediabetic drift.
Marked deficiency range< 11 nmol/L · < 317 ng/dLBelow ~10–11 nmol/L every symptom group becomes strong and clustered: weight gain and increased fat mass (especially abdominal), loss of muscle mass and strength, metabolic problems including type 2 diabetes and metabolic syndrome, loss of bone density (osteoporosis), depression and low mood, chronic fatigue, sleep problems, hot flushes, reduced cognitive performance — and severely reduced libido, erection strength and fertility. This is the range where the hypogonadal-obesity cycle takes over: low T drives fat gain, and fat aromatises testosterone away, deepening the deficiency.
Bhasin et al. · 2001
Five weekly doses. Five different outcomes.
In plain language: researchers first switched off the participants’ own testosterone production, then gave each group a different weekly dose for 20 weeks. At 25 mg, men lost lean mass and gained body fat. At 50 mg, testosterone remained very low at 306 ng/dL (10.6 nmol/L); symptoms of hypogonadism persisted, lean-mass change was not significant, and body fat also increased. Clear gains began at 125 mg, while 300 and 600 mg produced progressively larger gains in lean mass, muscle size and strength.
Bars show mean change in fat-free mass; the values below each dose show the study’s mean on-treatment total testosterone. The 300 and 600 mg groups were supraphysiological and are not treatment targets. The study demonstrates dose-response, not a recommended dose.
Bone substudy · Testosterone Trials
Strength was measured, not assumed.
Spine
Hip
Trabecular bone
One year of testosterone treatment increased volumetric bone mineral density and estimated strength in older men with low testosterone, with the largest changes in trabecular spine bone.
The trial measured density and estimated strength; it was not large or long enough to establish fewer fractures.
Dhindsa et al. · 2016
Insulin response improved by 32%.
In men with type 2 diabetes and hypogonadotropic hypogonadism, six months of testosterone improved directly measured insulin sensitivity and reduced subcutaneous fat and inflammatory markers.
HbA1c did not significantly change during this six-month trial. Improved insulin sensitivity is not the same as diabetes remission.
Questions & answers
Which benefits of TRT are best documented?
In men with confirmed low testosterone, trials show the most consistent effects on sexual desire and function and on body composition. Effects on energy and mood are smaller and vary between men.
Is TRT useful if my testosterone is normal?
The evidence for benefit comes from men with clearly low levels and symptoms. With normal levels the expected benefit is small, while the risks still apply.
What is monitored during treatment?
Typically testosterone levels, haematocrit, PSA depending on age, symptoms and side effects, at intervals agreed with the physician.