A testosterone result is not a fixed property of a person, like height. It is a measurement of a moving quantity, taken at one moment, using an assay with its own quirks. Get the conditions wrong and the number you receive is not a slightly worse estimate — it is noise you cannot interpret.
1. Time of day changes the answer
Testosterone peaks in the early morning and falls through the day, with the trough in the late afternoon. In younger men the difference between an 8am and a 4pm sample can be substantial. Every reference range you will be measured against was built on morning samples, so an afternoon result compared to a morning range is a category error.
2. One reading is not a diagnosis
Testosterone varies day to day in the same individual. Acute illness, a heavy training session, poor sleep and psychological stress all push it down temporarily. Guidance requires two samples on separate days before a diagnosis is made, precisely because single low readings so often fail to repeat.
3. Total testosterone alone answers very little
Most circulating testosterone is bound: tightly to SHBG, loosely to albumin. Only the unbound fraction — free testosterone, typically 1–2% of the total — is available to tissue.
That means two men with identical total testosterone can have very different free levels depending on their SHBG. SHBG rises with age, with liver disease and with hyperthyroidism; it falls with obesity, insulin resistance and hypothyroidism. Without it, a total testosterone figure cannot be placed in context.
| Man A | Man B | |
|---|---|---|
| Total testosterone | 11 nmol/L | 11 nmol/L |
| SHBG | 70 nmol/L (high) | 18 nmol/L (low) |
| Calculated free testosterone | Low | Normal |
| Likely interpretation | Symptoms plausibly explained | Look for another cause |
4. LH and FSH decide what happens next
These two pituitary hormones tell you whether the problem is the testes or the signal reaching them. It is the difference between a condition that is usually permanent and one that is often reversible. A testosterone test without LH and FSH cannot make that distinction.
5. Calculated free testosterone beats direct assays
Free testosterone can be measured directly by analogue immunoassay, or calculated from total testosterone, SHBG and albumin using the Vermeulen equation. Direct analogue assays are widely regarded as unreliable at the low end — exactly where the answer matters. Calculated free testosterone is the method guidelines favour.
What to ask for
- Total testosterone, morning sample
- SHBG and albumin, so free testosterone can be calculated
- LH and FSH
- Prolactin — a raised level points at a pituitary cause that needs proper investigation
- Oestradiol, if symptoms include breast tenderness or the picture is unclear
- A repeat of anything abnormal, on a different day
Sources
BSSM guidelines on adult testosterone deficiency, 2022
Morning sampling, the two-sample requirement and preference for calculated free testosterone.
Vermeulen A, Verdonck L, Kaufman JM (1999)
The calculation of free testosterone from total testosterone, SHBG and albumin — still the reference method.
