Biomarker 101: Testosterone and free testosterone

Testosterone comes with two numbers, and they can point in different directions. Understanding why is most of what makes a result readable.

What testosterone is

Testosterone is a sex hormone produced mainly in the testes, and in smaller amounts by the ovaries and adrenal glands. It contributes to muscle and bone maintenance, energy, mood, sexual function and fertility.

Its effects are more visible in men, but it matters in women too, at much lower concentrations.

Total, free and bioavailable

Testosterone does not dissolve well in blood, so most of it travels bound to carrier proteins. Around 95 per cent is attached to either sex hormone binding globulin (SHBG) or albumin. While bound, it is inactive.

Total testosterone measures everything — bound and unbound together.

Free testosterone is the small unbound fraction that can actually act on tissue. Measuring it directly is difficult, so it is usually calculated from total testosterone, SHBG and albumin.

Bioavailable testosterone sits between the two: free testosterone plus the loosely albumin-bound portion, which detaches readily enough to be usable. Testosterone bound to SHBG is held tightly and is not.

Why the two numbers can disagree

This is the practical consequence, and it works in both directions.

Someone can have a normal total testosterone with a low free testosterone, because SHBG is high — more of the hormone is locked up. SHBG rises with age, with an overactive thyroid, with liver conditions and with certain medications.

The reverse also happens: a low total with a normal free level, because SHBG is low. Lower SHBG is associated with obesity and insulin resistance.

In either case, reading total testosterone alone gives a misleading picture — which is why SHBG is measured alongside it.

Timing matters

Testosterone follows a daily rhythm, higher in the morning and declining through the day. Samples are generally taken in the morning for that reason, and a result from an afternoon sample is not comparable to one from 8am.

Which test to use

In men, total testosterone is generally the first-line measurement. Where it is borderline, or where it does not fit the clinical picture, free testosterone and SHBG add useful detail, often alongside LH and FSH to distinguish between possible causes.

In women, total testosterone and SHBG are usually assessed together, often expressed as the free androgen index, which describes the relationship between them.

Anyone taking prescribed hormone therapy will have monitoring directed by their prescribing clinician, including which markers are measured and when relative to their dosing schedule.

What testosterone testing won't tell you

• Total alone can mislead. Without SHBG, a normal total can conceal a low free level.
• Timing changes the answer. Morning and afternoon samples are not comparable.
• It isn't a diagnosis. Results are interpreted with symptoms, examination and other hormones.
• One reading isn't enough. Levels vary day to day, and borderline results are routinely repeated.

The bottom line

Testosterone is two measurements describing different things, connected by SHBG. Read together and taken in the morning, they give a clear picture. Read alone, total testosterone can point entirely the wrong way.