How SHBG Affects Free Testosterone

Sex hormone-binding globulin (SHBG) is the protein that locks up most of your testosterone, so the higher your SHBG, the less free — biologically usable — testosterone you have, even when total testosterone is identical. Two men can both read 20 nmol/L total testosterone and have very different free fractions purely because one carries twice the SHBG of the other.

TL;DR — key takeaways

• SHBG is a sponge. In men, roughly 45–65% of testosterone is bound tightly to SHBG and is not available to tissues; only about 2–3% is truly free.
• Total can lie. A normal total testosterone with high SHBG can still leave you with low free testosterone — the symptom-relevant fraction.
• The free index makes it visible. Free testosterone index (FTI) = (total testosterone ÷ SHBG) × 100, both in nmol/L. Double the SHBG and the index halves.
• It is an estimate, not a measurement. The FTI ignores albumin; the Vermeulen calculation uses albumin too and reports an actual free value in pmol/L.

What SHBG actually does to testosterone

Testosterone does not float freely in blood. The large majority is bound to two proteins: SHBG, which grips it tightly with high affinity, and albumin, which holds it loosely. Only the small unbound slice — free testosterone — can diffuse into cells and act. This is the “free hormone hypothesis”: the bound pool is a reservoir, and what your tissues feel is the free (and, to some extent, the loosely albumin-bound) fraction.

Because SHBG binds testosterone so tightly, it behaves like a sponge sitting in the bloodstream. When SHBG rises — with age, hyperthyroidism, liver disease, low body weight, or some medications — it soaks up a larger share of the same total testosterone, dropping the free fraction. When SHBG falls — with obesity, insulin resistance, or type 2 diabetes — less testosterone is bound and the free fraction climbs relative to total. That is why total testosterone alone can be misleading, and why the Endocrine Society advises measuring or calculating free testosterone in men whose total sits near the reference limits or whose SHBG is likely abnormal.

The number that captures it: the free testosterone index

The simplest way to see SHBG’s effect on paper is the free testosterone index (FTI), also called the free androgen index. It is a single ratio: total testosterone divided by SHBG, multiplied by 100, with both values in nmol/L.

The unit trap catches most people. SHBG is reported in nmol/L almost everywhere, but US labs often report total testosterone in ng/dL. You cannot divide ng/dL by nmol/L. Convert first: testosterone’s molecular weight is 288.4, so ng/dL ÷ 28.84 = nmol/L. Every example below is in nmol/L.

How SHBG bands map to the free index

Holding total testosterone fixed at 20 nmol/L, here is how rising SHBG drags the free index down. Reference ranges vary by lab and assay, so read this as a demonstration of the relationship, not a diagnosis.

Every row shares the same total testosterone. The only thing that changes is SHBG, and the index moves inversely — a clean illustration that free testosterone is a story about the binding protein as much as about the hormone.

The SHBG–free-T curve, visualised

Plot the free index against SHBG for a fixed total and you get a falling curve: steep at low SHBG, flattening as SHBG climbs. The first units of SHBG matter most.

Index vs the Vermeulen calculation

The free index is a proportional shortcut, not a true concentration. It uses only total testosterone and SHBG, so it tracks how SHBG shifts the free fraction but cannot give you free testosterone in pmol/L. The widely cited Vermeulen calculation adds albumin and the binding constants of SHBG and albumin to estimate an actual free value, and large comparisons by de Ronde and others show calculated free testosterone agrees reasonably with equilibrium dialysis when the right constants are used. For watching SHBG’s effect, the index is fine; for a number to compare against a free-testosterone reference range, prefer the Vermeulen estimate.

How this is calculated

Every figure on this page uses two facts only: the free index is total testosterone divided by SHBG times 100 in matched nmol/L units, and ng/dL converts to nmol/L by dividing by 28.84. There is no hidden constant and no assay involved — it is arithmetic on two lab numbers you already have. The calculators on this site automate the unit conversion and the division so you can sanity-check against the chart above. None of this is medical advice or a diagnosis; SHBG and free testosterone are interpreted by your clinician alongside symptoms and the rest of your panel.

Frequently asked questions