Your total testosterone came back at 12 or 15 nmol/L. Your GP says it's "in range." But you're tired all the time, your libido has dropped, your mood is flat, and your gym progress has stalled. Could low testosterone still be the cause?
Yes — and the key word is free testosterone.
Total vs Free Testosterone: What's the Difference?
Around 60% of testosterone in the blood is tightly bound to sex hormone-binding globulin (SHBG) — and this fraction is biologically inactive. Another 38% is loosely bound to albumin. Only 2–3% is "free" testosterone, unbound and available to act on your cells.
When SHBG is high, it mops up a larger proportion of circulating testosterone — leaving your free (active) fraction far below where it should be, even when total testosterone appears normal.
What Causes High SHBG?
- Age — SHBG rises progressively from the mid-30s
- Thyroid disease (hyperthyroidism raises SHBG significantly)
- Liver disease or elevated liver enzymes
- Low oestrogen (paradoxically, high SHBG can occur in both extremes)
- Very low body fat or severe caloric restriction
- Certain medications — anticonvulsants, some statins
The Panel You Actually Need
| Test | Purpose | Target (Adult Men) |
|---|---|---|
| Total testosterone | Starting point — but rarely the whole picture | 9–35 nmol/L |
| SHBG | Key determinant of bioavailable fraction | 18–54 nmol/L (lower is better for symptoms) |
| Free testosterone (calculated) | The number that correlates with symptoms | >0.25 nmol/L |
| LH + FSH | Determines whether the issue is the pituitary or the testes | LH 1.5–9.3 IU/L |
| Oestradiol | Elevated oestradiol further suppresses free T and worsens symptoms | 80–200 pmol/L |
| Prolactin | Hyperprolactinaemia suppresses LH and testosterone | <450 mIU/L |
| Thyroid (TSH) | Hypothyroidism and hyperthyroidism both affect SHBG | 0.4–4.0 mIU/L |
Does High SHBG Mean I Qualify for TRT?
A low free testosterone in the context of clear symptoms is a legitimate basis for a TRT discussion — even if total T is within range. Guidelines from the British Society for Sexual Medicine and the European Association of Urology both acknowledge calculated free testosterone as a valid diagnostic criterion. A private GP specialising in testosterone can assess your full clinical picture and advise accordingly.
Full Testosterone Assessment at Lambert Medical Practice
We run the complete panel — total testosterone, SHBG, free testosterone, LH, oestradiol, prolactin, and thyroid — and interpret results in context of your symptoms.
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