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Reproductive Health

Free Testosterone

Free testosterone is the small, active part of your testosterone that your body can actually use. Most of your testosterone is gripped onto a carrier protein in your blood and held in reserve, so how much is free depends on how much of that carrier protein you have. That is why this number is read together with your other testosterone results, not on its own.

Targeted Testing
CategoryReproductive Health
Reference rangeSee your report
Evidence○Targeted Testing
Last updated29 June 2026
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What is free testosterone?

Most testosterone in your blood is attached to proteins — mainly sex hormone binding globulin (SHBG), a carrier protein made by the liver — and is held in an inactive form. Only the fraction that is not bound to these proteins is immediately available to your tissues and cells. This is free testosterone. Studies show that only about two per cent of the total testosterone in circulation is truly unbound and immediately active. Because free testosterone is the biologically active portion, it can give a clearer picture of how much testosterone is actually reaching your cells, particularly when SHBG levels are unusual — for example, when ageing, illness, or certain medicines shift the balance between bound and free hormone.

Why is it tested?

Free testosterone is often measured alongside SHBG (sex hormone binding globulin — the main carrier protein) when there is reason to think the ratio of bound to free testosterone may be off. Conditions that can shift SHBG — and therefore the free fraction — include obesity, thyroid disease, ageing, liver disease, HIV, nephrotic syndrome (a kidney condition), and some medicines. In these situations, total testosterone alone can be misleading because a normal-looking total may conceal a low free fraction, or vice versa. Free testosterone can be measured directly in a blood sample or calculated mathematically from total testosterone and SHBG — the calculated approach is often used where SHBG variations are suspected.

Reference range (Australia)

No single 'normal' value — free testosterone is method-dependent and varies by sex and age; use the reference interval printed on your report pmol/L

Free testosterone is either measured directly or calculated from total testosterone and SHBG. The two approaches can give materially different numbers, so the method matters. The reference interval varies by sex, age, and laboratory method. Always use the interval printed on your own report, which reflects your laboratory's specific approach.

Reference intervals vary between laboratories. Always use the range printed on your own report.

What your result means

If your level is low

A low free testosterone result means less active hormone is reaching your cells. This can reflect lower total testosterone production, a high SHBG (which binds more of the available testosterone), ageing, or certain underlying conditions. Your GP will read it alongside your total testosterone and SHBG to work out which part of the system is driving the result.

If your level is in range

A result within the reference interval for your laboratory suggests the active testosterone fraction is in the expected range for your sex and age. Because the method used to measure or calculate free testosterone varies between laboratories, the interval on your own report is the relevant guide.

If your level is high

A raised free testosterone result means more active hormone is reaching your cells. What it means depends on your sex, age, and the wider picture. In women, a higher free testosterone can go along with androgen excess — where the body makes or responds to more male-type hormone than usual. On its own this is a pattern to look into with your GP, not a diagnosis. It is read together with your total testosterone, SHBG and symptoms, never from one number. In men, the result is read in the context of total testosterone and SHBG. A single result does not settle the picture on its own.

What can affect your result

Free testosterone is shaped by two things: how much total testosterone your body makes, and how much SHBG is present to bind it. Anything that raises SHBG will lower the free fraction — ageing is a major driver, since SHBG rises gradually over time, pulling more testosterone into the bound (inactive) form. Conditions that lower SHBG (and so raise free testosterone relative to total) include obesity and insulin resistance — where the body responds less well to its own insulin. Thyroid disease, liver disease, and some medicines can shift SHBG in either direction. Oral contraceptives (the pill) tend to raise SHBG, which lowers the free fraction. Calculations of free testosterone from total testosterone and SHBG are a practical approach when SHBG variation is suspected — your GP can explain which method your laboratory used.

When to act

If your free testosterone is outside the reference interval, raise it with your GP alongside your total testosterone and SHBG results — these three numbers are read together, not in isolation. Your GP can work out whether the issue lies with how much testosterone is being made or with how much is being bound by carrier protein, and advise whether any follow-up is needed. This page is general information and is not a substitute for advice from your GP.

Frequently asked questions

Total testosterone is the combined measurement of all testosterone in your blood — both the fraction bound to carrier proteins and the small free fraction. Only the free, unbound portion (roughly one to three per cent of the total) is biologically available and immediately active in your tissues. If SHBG (the main carrier protein) is unusually high or low, your total testosterone can look normal while the free fraction — the amount your cells can actually use — is quite different. That is why both are often measured together.

SHBG (sex hormone binding globulin — the liver protein that carries testosterone in the blood) largely determines how much testosterone is free and active. When SHBG is affected by conditions such as obesity, thyroid disease, ageing, liver disease, or certain medicines, total testosterone alone can give a misleading picture. Measuring SHBG at the same time allows a more accurate assessment of how much testosterone is actually reaching your cells.

Several conditions can shift SHBG — the carrier protein that determines how much testosterone is free. Ageing raises SHBG, which means more testosterone is bound and less is free, so the active fraction falls even when total testosterone stays in range. Obesity and insulin resistance tend to lower SHBG. Thyroid disease, liver disease, HIV, and some kidney conditions (such as nephrotic syndrome) can all alter SHBG, which is why free testosterone is sometimes tested alongside total testosterone in these situations.

There is no single universal number. Free testosterone is method-dependent — whether it is directly measured or calculated from total testosterone and SHBG makes a real difference to the result — and it varies by sex, age, and the laboratory's reference interval. The relevant comparison is always the interval printed on your own report. Your GP uses that interval to assess your result in the context of your total testosterone and SHBG.

Both approaches are used. Free testosterone can be calculated mathematically from total testosterone and SHBG — a practical approach especially when SHBG is known to vary. It can also be measured directly in a blood sample. The two methods can produce different numbers, so it matters which one your laboratory used. Your lab report will indicate the method; your GP can explain what the result means for you.

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