Free vs Total Testosterone and SHBG: An Australian Guide

If you have had your testosterone checked, you may have noticed the result is not as simple as a single number. Your pathology report can list total testosterone, free testosterone, SHBG and sometimes bioavailable testosterone — and it is entirely possible to have a "normal" total testosterone while still feeling the effects of a low active level, or the other way around. Understanding what each of these measurements means explains why one figure rarely tells the whole story, and why a sound diagnosis rests on more than a single blood test. It also explains why a GP may want to look beyond the total figure before drawing any conclusions.

Total testosterone: the number most people see

Total testosterone measures every molecule of testosterone circulating in your blood, whether it is bound to a protein or floating free. In Australia it is reported in nanomoles per litre (nmol/L), and it is the standard first-line test when low testosterone is suspected. Australian and international guidelines both recommend measuring it in a morning, fasting blood sample, because testosterone follows a daily rhythm and eating can temporarily lower the reading (Endocrine Society guideline; ESA position statement). The catch is that most of the testosterone counted in that total is not actually available to your tissues.

Free and bioavailable testosterone: the active fraction

Once testosterone enters the bloodstream, a large proportion binds tightly to a protein called sex hormone-binding globulin (SHBG). Much of the remainder binds loosely to albumin, and only a small fraction — around 1–2% — circulates completely free. It is this free testosterone, together with the loosely albumin-bound portion, that can readily act on your tissues.

• Free testosterone — unbound and biologically active.
• Bioavailable testosterone — the free fraction plus the loosely albumin-bound fraction; broadly, the testosterone your body can actually use.
• SHBG-bound testosterone — held tightly and not readily available.

In day-to-day terms, it is the free and bioavailable testosterone — not the portion tied up on SHBG — that drives most of the effects men associate with the hormone. This is why two men with an identical total testosterone can carry very different amounts of active hormone; the difference often comes down to SHBG.

SHBG: the protein that changes the picture

SHBG is produced by the liver, and how much you have shifts the balance between total and free testosterone. When SHBG is high, more testosterone is locked away and less is free; when SHBG is low, a greater share of your total is available. Several common factors push it one way or the other:

• Lower SHBG — obesity, insulin resistance and type 2 diabetes, and an underactive thyroid. These can pull the total reading down even when the free fraction is less affected.
• Higher SHBG — ageing, liver disease, an overactive thyroid, oestrogen and some medications. These can prop the total reading up while the free fraction is lower than it appears.

Because of this, both Australian and international guidelines advise measuring free testosterone when the total sits near the lower end of the range, or when a man has a condition known to alter SHBG (Endocrine Society guideline).

Why a "normal" total testosterone can mislead

This is where the distinction matters most in practice.

The older man with high SHBG. SHBG tends to rise with age, so an older man can return a total testosterone that looks reassuringly normal while his free — and therefore active — testosterone is genuinely low. Reading the total alone could miss it.

The man with low SHBG. In someone carrying excess weight or living with type 2 diabetes, SHBG is often low. The total may look low while the free fraction is relatively preserved, so the total in isolation can overstate the problem. In both cases, the free fraction fills in what the total leaves out.

The European Male Ageing Study reflected this by defining age-related low testosterone using both measures together — a combination of sexual symptoms alongside a total testosterone below roughly 11 nmol/L and a calculated free testosterone below about 220 pmol/L (European Male Ageing Study). Neither number was read on its own.

Calculated free testosterone and what Australian labs report

Measuring free testosterone directly is not straightforward. The reference-standard method, equilibrium dialysis, is accurate but impractical for routine use, while the cheaper direct "analogue" immunoassays are widely regarded as unreliable. Guidelines therefore favour calculated free testosterone — an estimate derived from your total testosterone, SHBG and albumin (Endocrine Society guideline). It is not a separate blood draw; the same sample that gives your total testosterone and SHBG is used to work it out.

In practice, many Australian pathology laboratories report total testosterone and SHBG, and either include a calculated free testosterone or provide it on request. Reference ranges differ between laboratories and assays, which is one reason your GP interprets the results against your symptoms rather than a single universal cut-off. Our guide to the blood tests used to investigate low testosterone covers what a full work-up usually includes.

Getting an accurate result: timing and repeat testing

Because testosterone varies through the day and from day to day, when and how the sample is taken matters as much as the numbers. Guidelines recommend:

• A morning sample, ideally before late morning, when testosterone is at its daily peak.
• Fasting, since eating can transiently lower the result.
• At least one repeat test on a separate day, because a meaningful proportion of men with a low initial reading return a normal level when it is checked again (Endocrine Society guideline; ESA position statement).

Just as importantly, low testosterone is a clinical diagnosis rather than a number below a line. Australian guidance is clear that deficiency should be diagnosed only when consistent symptoms of low testosterone are matched by low readings on more than one occasion — not on a single borderline result (ESA position statement). Because the reference range also shifts across the lifespan, it helps to understand how normal testosterone levels change with age.

The bottom line

"Testosterone" on a blood test is not one number but several. Total testosterone captures everything in the blood, SHBG determines how much is locked away, and the free and bioavailable fractions are what your body can actually use. A normal-looking total can hide a low free level in a man with high SHBG, while a low total can overstate the problem when SHBG is low. That is why a careful assessment uses a morning, fasting sample, looks at SHBG and free testosterone, repeats the test, and — above all — reads the numbers alongside your symptoms. If your results and how you feel do not seem to line up, that mismatch is worth discussing with an AHPRA-registered GP.

References

1. Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744. https://pubmed.ncbi.nlm.nih.gov/29562364/
2. Yeap BB, et al. Endocrine Society of Australia position statement on male hypogonadism (part 1): assessment and indications for testosterone therapy. Med J Aust. 2016;205(4):173–178. https://www.mja.com.au/journal/2016/205/4/endocrine-society-australia-position-statement-male-hypogonadism-part-1
3. Wu FCW, et al. Identification of Late-Onset Hypogonadism in Middle-Aged and Elderly Men. N Engl J Med. 2010;363(2):123–135. https://pubmed.ncbi.nlm.nih.gov/20554979/

This article is general information only and is not a substitute for personalised medical advice. Testosterone is a prescription-only (S4) medicine in Australia and is prescribed only where clinically appropriate. Speak with an AHPRA-registered GP about your individual situation.