Total vs free testosterone: what the numbers really mean

The short answer

Total testosterone is the combined amount of testosterone circulating in your bloodstream. The vast majority of it is bound to two carrier proteins — SHBG (sex hormone binding globulin) and albumin — and cannot act on your tissues while it is bound. Free testosterone is the tiny unbound fraction, typically 1–3% of the total, that is biologically active. Because SHBG varies widely between individuals, two men with identical total testosterone can have very different free testosterone, and therefore very different symptoms.

Why free testosterone can matter more

Only the free (and loosely albumin-bound) fraction can cross cell membranes and bind to androgen receptors. That means it is free testosterone, not total, that drives libido, energy, mood, red-blood-cell production and the maintenance of muscle and bone. If SHBG is high — common with ageing, thyroid overactivity, liver disease, certain anticonvulsants and oestrogen exposure — more testosterone is locked away and the free fraction falls, even when total looks normal on paper. The opposite happens when SHBG is low (obesity, insulin resistance, type 2 diabetes, hypothyroidism): free testosterone can be preserved despite a modest total.

This is why UK guidance from the British Society for Sexual Medicine (BSSM, 2022) recommends calculating free testosterone from total testosterone, SHBG and albumin whenever a man's total sits in the lower half of the reference range (roughly 8–12 nmol/L) or symptoms suggest deficiency. The calculation — usually the Vermeulen equation — is considered more reliable than direct free testosterone immunoassays, which can be inaccurate.

How to interpret your result

UK laboratories typically report total testosterone in nmol/L and calculated free testosterone in pmol/L (occasionally nmol/L). Broadly, adult men fall in these ranges, although your own lab report's interval is the one that matters:

• Total testosterone: about 8.6 to 29.0 nmol/L, narrowing with age.
• SHBG: about 18 to 54 nmol/L in adult men.
• Calculated free testosterone: about 200 to 620 pmol/L (0.20 to 0.62 nmol/L).

A total consistently below 12 nmol/L on two morning samples, combined with symptoms, triggers further evaluation. Below 8 nmol/L usually warrants referral. Between 8 and 12 nmol/L, calculated free testosterone below approximately 225 pmol/L is often considered clinically low. These are thresholds for discussion with a qualified clinician, not self-diagnosis — one-off readings can be knocked around by illness, poor sleep, alcohol, medications and even the time of day.

Choosing a private test that reports both

A useful private hormone panel for men should include, as a minimum, total testosterone, SHBG and albumin — the three ingredients needed to calculate free testosterone. More comprehensive panels add LH, FSH, oestradiol, prolactin and DHEA-S, which help clinicians work out whether a low result originates in the testes or the pituitary. Look for laboratories accredited by UKAS to ISO 15189 and clinics regulated by the Care Quality Commission. Every provider listed on myhealth checkup meets these criteria.

Timing matters more than most patients realise. Testosterone peaks in the early morning and can drop by 20–30% by the afternoon. UK clinical pathways expect samples between 07:00 and 11:00, ideally fasted, with any low or borderline result confirmed on a second morning draw before decisions are made.

Frequently asked questions