The panel / Androgens

Free testosterone

Free testosterone is the fraction your tissue can actually use, and we draw it on every patient. The catch is not whether to measure it but how. The cheap version of this test is the one the Endocrine Society says outright is inaccurate and should not be used, and it is still what most walk-in panels report.

What it measures

Only about half a percent to two percent of your testosterone circulates unbound. That unbound fraction is what reaches receptors. Free testosterone tracks symptoms, bone density and blood counts more closely than the total does. A related figure, bioavailable testosterone, adds the loosely albumin-bound share on the grounds that it comes off during transit through tissue.

The unbound fraction, roughly 0.5 to 2 percent of total testosterone

Correlates with symptoms more closely than total testosterone does

Bioavailable testosterone means free plus albumin-bound

Whether this number changes your diagnosis depends on your SHBG. When SHBG is normal it largely confirms what the total already showed. When SHBG is abnormal it becomes the number that decides. We draw it either way, because you do not know which situation you are in until it is measured.

How it is measured

SpecimenSerum, drawn with total testosterone and SHBG

TimingMorning, roughly 7 to 10am

FastingYes, glucose lowers testosterone

Best methodEquilibrium dialysis, the reference standard

Practical methodCalculated from total testosterone, SHBG and albumin

Never useDirect analog immunoassay

Three methods exist and they are not equivalent. Equilibrium dialysis physically separates the free fraction and is the reference standard, but it is expensive and not widely available. The calculated value, using the Vermeulen equation, is the practical workhorse and holds up well, though it runs about 19 percent higher than dialysis. The direct analog immunoassay does not truly measure the free fraction at all, and current guidance is explicit that it should not be used. If a previous report does not state which method produced the number, that is worth knowing before it is acted on.

Why it is on the panel

We measure it on every patient at the first visit, alongside SHBG. There are two situations where it changes the answer outright. The first is anything that shifts SHBG, because that pulls the total away from the free fraction in either direction. The second is a borderline total, roughly 200 to 400, where the total alone cannot decide. In the remaining cases it confirms rather than reveals, which is still worth having: a confirmed picture is what lets us tell you the answer is somewhere else on the panel and mean it.

Interpretation

Reading the result

Reference ranges for free testosterone are less standardised than for total testosterone, and the range printed on your report is the one that applies to your sample.

Bands used in practice

Below 50 to 65 pg/mLCommonly cited lower limit, though not firmly established

Normal SHBG, normal totalFree testosterone adds nothing here

High SHBG, normal totalThis is where a genuinely low free result is found

Low SHBG, low totalThis is where a normal free result rules deficiency out

The pattern matters more than the number. A low free testosterone alongside a normal total, in a man with high SHBG, is real deficiency that a total-only panel would have missed entirely. A normal free testosterone alongside a low total, in a man with obesity or diabetes, is not deficiency at all and should not be treated as such.

What moves the number without any disease being present

Most of these act by moving SHBG rather than by changing how much testosterone you make.

8 things that move it

Age. Free testosterone declines steadily, driven both by a falling total and a rising SHBG.

Body weight. The relationship is not linear. A BMI of 30 lowers measured free testosterone by around 14 percent, and a BMI of 35 by around 22 percent.

Time of day and food. Inherited from the testosterone draw, so the same conditions apply.

Alcohol. Sustained or binge intake raises SHBG, which lowers the free fraction.

Anticonvulsants. Carbamazepine, phenytoin and phenobarbital raise SHBG substantially.

Opioids and glucocorticoids. Both suppress the axis directly.

Low albumin. Most calculators assume a fixed albumin value, so liver or kidney disease introduces error.

Assay quality. The calculation is only as good as the total testosterone and SHBG fed into it, and SHBG assays are not fully standardised between laboratories.

What happens next

If free testosterone is low and the symptoms fit, LH and FSH follow, to separate a testicular cause from a pituitary one.

If free testosterone is normal despite a low total, the answer is usually a low SHBG from weight or insulin resistance. That is addressed directly rather than with testosterone.

If a previous result came from a direct analog immunoassay, it does not carry over and is repeated properly.

The baseline is carried forward, so that later results are read against your own starting point.

Straight answers

Both, and the combination is itself the finding. It usually means your SHBG is low, which is common with excess weight, insulin resistance or type 2 diabetes. A low SHBG drags the total down while leaving the usable fraction intact. Treating that with testosterone would be treating a number rather than a person. Weight, sleep and glucose are where the work is.