Men’s health › Reading a testosterone test
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The short version. One testosterone number means much less than it looks like it means. It moves through the day, it moves between labs, and it depends on a binding protein most reports do not measure. That is why both major guidelines require two fasting morning draws before anyone calls it a diagnosis — and why about 30% of borderline first results come back normal on the repeat.
Total, free, and why the difference matters
Most testosterone in your blood is bound — largely to sex hormone-binding globulin (SHBG), partly to albumin. Only a small unbound fraction is free. Total testosterone measures everything. Free testosterone measures the unbound portion.
The AUA white paper is clear on the starting point: “TT is the recommended initial test to diagnose hypogonadism.” Total first.
The Endocrine Society adds when to go further: clinicians should determine free testosterone “either directly from equilibrium dialysis assays or by calculations” in men with conditions that alter SHBG, or when total testosterone sits near the lower limit of normal.
What moves SHBG
This is where a total-only result can mislead. From the AUA white paper:
| Raises SHBG → total looks higher | Lowers SHBG → total looks lower |
|---|---|
| Ageing | Obesity |
| Hyperthyroidism | Diabetes |
| Oestrogens | Hypothyroidism |
| HIV disease | Glucocorticoids |
| Anticonvulsants | Androgenic steroids |
| Cirrhosis, hepatitis | Nephrotic syndrome, acromegaly |
The white paper draws out the consequence: “The increase in SHBG with age means that older men may have a normal TT levels, even if they are hypogonadal.” An older man with a reassuring total may still be deficient in the fraction that does the work.
Why the morning, and why fasting
Testosterone follows a daily rhythm. The AUA white paper: “Serum T levels peak in the early morning… Nadir values are approximately 15% lower than morning values.”
Fifteen per cent is enough to move a man across a decision threshold purely by choosing an afternoon appointment. Both guidelines therefore specify early morning, fasting, and — critically — twice. The Endocrine Society: “clinicians should measure total testosterone concentrations on two separate mornings when the patient is fasting.”
The lab variability nobody mentions
Reference ranges are assay-specific and lab-specific. Cleveland Clinic states it plainly: “Laboratories may have different reference ranges for normal testosterone levels.”
How much difference? The AUA white paper cites work by Vesper and colleagues in which a standard sample with a true value of 299 ng/dL was sent to eight reference laboratories. The results came back between 198 and 364 ng/dL.
That range straddles the AUA’s 300 ng/dL threshold in both directions. The same blood could be reported as clearly deficient or comfortably normal depending on which lab ran it. The white paper draws the obvious conclusion: “We believe that rigid use of T cut-off (300 ng/dL) may lead to unnecessary treatment of asymptomatic men as well as under-treatment of men with persistent signs and symptoms.”
LH and FSH: where the problem is
If testosterone is genuinely low, the next question is which part of the system has failed. The Endocrine Society: “We recommend distinguishing between primary (testicular) and secondary (pituitary-hypothalamic) hypogonadism by measuring serum luteinizing hormone and follicle-stimulating hormone concentrations.”
| Testosterone | LH / FSH | Interpretation |
|---|---|---|
| Low | High | Primary — the testes are not responding; the pituitary is shouting louder |
| Low | Low or inappropriately normal | Secondary — the signal from the pituitary or hypothalamus is inadequate |
The distinction changes what happens next: secondary hypogonadism can have a treatable cause, and it matters for fertility planning. AUA Statement 6 requires LH measurement in men with low testosterone. A service that never orders it has skipped a guideline step.
What a complete workup looks like
- Symptoms and signs documented first — the number alone is not a diagnosis.
- Total testosterone, early morning, fasting.
- A second total testosterone, another morning, also fasting.
- Free testosterone or SHBG where total is borderline or an SHBG-altering condition is present.
- LH and FSH if testosterone is confirmed low.
- Haemoglobin and haematocrit before any treatment starts (AUA Statement 11).
- PSA and prostate assessment as appropriate to age and risk.
Sources
- Bhasin S, Brito JP, Cunningham GR, et al., Endocrine Society Clinical Practice Guideline, J Clin Endocrinol Metab 2018;103(5):1715–1744 — academic.oup.com
- American Urological Association, “Evaluation and Management of Testosterone Deficiency: AUA Guideline” — auanet.org
- Paduch DA, et al., “The Laboratory Diagnosis of Testosterone Deficiency”, AUA white paper — auanet.org
- Cleveland Clinic, “Testosterone Test”, last reviewed 9 September 2024 — my.clevelandclinic.org
More in the men’s health hub: what TRT is · ED causes and treatments
Medical disclaimer. This article is for general information only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any condition. Interpreting a laboratory result requires a clinician who knows your history. Talk to your own healthcare provider before starting, stopping or changing any medication. Individual results vary.
