Testosterone
A key sex hormone for muscle, bone, mood, energy and libido.
Testosterone is a hormone present in everyone, at much higher levels in men than in women. It influences muscle and bone mass, red blood cell production, mood, energy and libido, and in men it is made mainly by the testes in response to signals from the brain.
Most of what circulates is not available to your tissues. The majority is bound tightly to sex hormone binding globulin, some is loosely bound to albumin, and only a small free fraction is active, which is why a total testosterone result can look reassuring while the free level does not, and why SHBG is measured alongside it.
It matters because genuinely low testosterone shows up as fatigue, low mood, reduced libido and difficulty holding muscle. It also matters because those symptoms overlap almost entirely with depression, poor sleep, thyroid disease and simply being run down, which is how treatable problems get mislabelled.
Diagnosis is deliberately slow for that reason. Clinical guidance requires symptoms plus repeated morning fasting measurements, because levels peak in the morning, swing between days, and fall temporarily with illness, short sleep and heavy training. One low reading from an afternoon finger prick test is not a diagnosis of anything.
The over the counter booster category does not work. An analysis of the most popular online testosterone boosting supplements found 19 distinct ingredients across the top five products; of 191 studies on the ten commonest ingredients, only 19% involved humans at all, and among those human studies 30% showed an increase, 46% showed no effect and 3% showed a decrease. The customer reviews told a better story than the evidence did.
The levers that do move it are unglamorous and free: sleeping properly, reducing heavy drinking, and losing visceral fat if you are carrying it. Sleep restriction lowers testosterone measurably in controlled studies, and obesity and alcohol are well established suppressors.
Prescribed testosterone therapy is a clinical decision with real consequences, including suppression of your own production and of sperm output, and it belongs with a doctor rather than with a website. Recovery of fertility after stopping is common but can take many months and is not guaranteed.
Sources
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
Men
- Below typical
- under 6 nmol/L
- Typical range
- 6 to 27 nmol/L
- Above typical
- 27 nmol/L and over
Men only, and MORNING SAMPLES ONLY: testosterone falls through the day, so an afternoon draw can read low in someone whose morning level is unremarkable. Results also vary meaningfully between laboratory methods. For women the interval depends on cycle stage and menopausal status, so none is drawn. Reference intervals differ between laboratories and assays. Where your own report prints a range, the dashboard draws that one instead of this.
Reference interval from North Bristol NHS Trust, Severn Pathology. Read against that page on 2026-08-09. Your own laboratory may print a slightly different range.
On your own blood test?
Testosterone is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, your own laboratory’s printed range is used in place of the one above wherever your report carries one, and your results are explained together in one place.