Free Testosterone

Hormones & Labs · Reviewed August 2026

Definition

Free testosterone is the small share of testosterone that circulates in blood unbound to carrier proteins. Most testosterone travels attached to SHBG or albumin, and only about 1% to 4% is free.[1] Labs report free testosterone in picograms per milliliter (pg/mL), usually calculated from total testosterone and SHBG rather than measured directly.[2]

Educational content, not medical advice. For personal or fertility concerns, please consult your doctor.

What Free Testosterone Measures

Testosterone doesn't float through the bloodstream on its own. Most of it circulates bound to plasma proteins, mainly SHBG (sex hormone-binding globulin) and albumin.[5] SHBG grips testosterone tightly. Albumin holds it loosely. What's left over, roughly 1% to 4%, is the free fraction.[1] Under the free hormone hypothesis, that unbound fraction is the part considered biologically active in target tissues.[1] You'll also see the term "bioavailable testosterone" on some panels, which combines the free fraction with the loosely albumin-bound share.[1]

So free testosterone answers a different question than total testosterone. Total counts everything in the tube, bound and unbound. Free counts only the small unbound slice.

How It's Measured

The reference method is equilibrium dialysis, which physically separates the free hormone before measuring it.[2] It's accurate but slow, so it mostly lives in research and specialty labs. Most clinical labs report calculated free testosterone instead. They measure total testosterone and SHBG, then run both through a published equation. In the classic validation study, calculated values were almost identical to equilibrium dialysis under all conditions studied except pregnancy.[2] A third approach, the direct analog immunoassay, performed poorly in that same study. Its results varied with SHBG levels and came out as only a fraction of the reference values.[2]

On a lab report, free testosterone usually appears as its own line under the total testosterone result, in pg/mL. The total testosterone test is the most common of the three, with free and bioavailable ordered as add-ons.[4]

Why People Track Free Testosterone

Because the free fraction can move on its own. SHBG and the other binding proteins regulate how much of a circulating steroid hormone stays free.[3] When SHBG rises, more testosterone gets bound and the free share shrinks. When SHBG falls, the free share grows. Two lab draws can show the same total testosterone and still carry different free testosterone, just because SHBG changed between them. That's why hormone panels often report total testosterone, free testosterone, and SHBG together. The three numbers only tell their story as a set.

For how these panels fit alongside the other hormones people test, see At-Home Hormone Tests, Explained.

How Go Go Gaia Tracks Free Testosterone

Free testosterone is a lab value, so it comes in through lab upload. Add a lab report PDF or CSV to the web app and Go Go Gaia pulls out the Free Testosterone result, stores it in pg/mL, and charts it on your timeline. Because the app also stores total testosterone and SHBG from the same reports, you can see all three lines over time instead of one number in isolation.

See your hormone panel as one picture

Upload lab results and chart free testosterone next to total testosterone, SHBG, and everything else you track.

Download Go Go Gaia

Free to download on the App Store.

What the Research Shows

A 2017 review in Endocrine Reviews reappraised how testosterone binds in circulation and notes that the free hormone hypothesis, the idea that only the unbound fraction acts on tissues, has been debated extensively and that the standard models of binding are under active revision.[1] Free testosterone is a well-used measurement built on a still-evolving model.

The 1999 validation study behind calculated free testosterone compared simple estimation methods against equilibrium dialysis and concluded that the calculation from total testosterone and SHBG is a rapid, simple, and reliable index of bioavailable testosterone.[2] That finding is why the calculated version became what most labs report.

Work on the binding proteins themselves describes SHBG and its relatives as gatekeepers of steroid hormone action, actively regulating the free fractions of circulating steroids rather than just carrying them around.[3]

Related Terms

Testosterone

The total measurement this entry's free fraction is carved out of.

SHBG (Sex Hormone-Binding Globulin)

The carrier protein that sets how much testosterone stays free.

DHEA-S

An adrenal androgen often reported on the same panel as testosterone.

Estradiol

Another SHBG-bound steroid hormone, and a standard hormone-panel line.

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

What's the difference between total and free testosterone?

Total testosterone counts everything in the blood, both the protein-bound share and the free share. Free testosterone counts only the small unbound fraction, roughly 1% to 4% of the total, which is the portion considered biologically active under the free hormone hypothesis.

Is free testosterone measured directly or calculated?

Both exist. Equilibrium dialysis measures it directly and is the reference method, but it's slow and mostly lives in research and specialty labs. Most clinical labs instead calculate free testosterone from total testosterone and SHBG using published equations, and those calculated values track the dialysis method closely.

Why can free testosterone change when total testosterone stays the same?

Because SHBG sets how much of the total is bound. SHBG and other binding proteins regulate the free fraction of circulating steroid hormones, so when SHBG rises more testosterone gets bound and the free share shrinks, and when SHBG falls the free share grows, even if total testosterone hasn't moved.

References

  1. Goldman AL, Bhasin S, Wu FCW, Krishna M, Matsumoto AM, Jasuja R. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocrine Reviews. 2017;38(4):302–324. pubmed.ncbi.nlm.nih.gov/28673039
  2. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. The Journal of Clinical Endocrinology & Metabolism. 1999;84(10):3666–3672. pubmed.ncbi.nlm.nih.gov/10523012
  3. Hammond GL. Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action. Journal of Endocrinology. 2016;230(1):R13–R25. pubmed.ncbi.nlm.nih.gov/27113851
  4. National Library of Medicine. Testosterone Levels Test. MedlinePlus. medlineplus.gov/lab-tests/testosterone-levels-test
  5. Nassar GN, Leslie SW. Physiology, Testosterone. StatPearls. StatPearls Publishing. ncbi.nlm.nih.gov/books/NBK526128

Sourced from peer-reviewed research and, for basic mechanism claims, established references like NIH and StatPearls. Every citation above is checked to confirm it resolves to a real source and supports the sentence it's attached to. Last reviewed August 2026. See our full methodology on the Encyclopedia hub.