Free Testosterone
The ~1–2% of testosterone unbound to SHBG or albumin — the only fraction that can enter cells and activate androgen receptors.
This is usually the number that explains your symptoms. Two people with identical total T can feel completely different depending on SHBG. It's why someone with 'perfectly normal' total testosterone can still have every symptom of deficiency.
Check a Free Testosterone result against this range →
What Free Testosterone actually measures — the analyte, and the assay
Three completely different things are sold under this name, and the report usually does not say which one you bought. Equilibrium dialysis separates the unbound hormone across a membrane and measures what crosses, ideally by LC-MS/MS. A calculated free testosterone is not measured at all — it is total testosterone, SHBG and albumin put through a binding equation. An analog or direct free testosterone immunoassay uses a labeled analog intended to ignore the bound fraction, and it is the cheapest and the worst Kacker 2013.
Dialysis is the reference and it is still not simple. Free testosterone calculated from a direct equilibrium-dialysis measurement forced a reassessment of the binding constants the calculations use, because the classical model assumes one high-affinity SHBG site and the real binding does not behave that way Fiers 2018 Goldman 2017.
Which equation was used changes the answer. Accuracy of calculated free testosterone differs between published equations and the disagreement depends on sex and on SHBG concentration Hackbarth 2011. So two laboratories can start from identical total testosterone, SHBG and albumin values and print different free testosterones, entirely because of the algebra.
The direct immunoassay is the one to recognize on sight. Its analytical performance against the Vermeulen calculation has been examined in clinical series and it does not track Dalmiglio 2024 Kacker 2013. A free testosterone reported in pg/mL from a routine panel with no SHBG beside it is usually this one.
Free Testosterone: what changes the blood, and what only changes the reading
What changes the free hormone in you:
- Total testosterone, which is the numerator and carries every one of its own influences into this number.
- SHBG, the denominator. It falls with obesity, insulin resistance, high insulin, androgens and glucocorticoids; it rises with aging, thyroid hormone excess, estrogen, and liver disease Szybiak-Skora 2025. A halving of SHBG roughly doubles the free fraction with no change in the gonad at all.
- Albumin, the second binder, which drops in inflammation, nephrotic syndrome and liver failure and takes a large slice of the loosely bound pool with it Goldman 2017.
- Anything that displaces the hormone from its binder — a high non-esterified fatty acid load, some drugs.
What changes only the reading:
- The equation. Vermeulen, Sodergard, Ekins and the linear approximations do not agree, and the disagreement is largest exactly where SHBG is unusual Hackbarth 2011 Fiers 2018.
- The albumin the calculator assumed. Many calculators default to 4.3 g/dL. If yours is 3.6 the printed free testosterone is wrong and nothing about you changed.
- The analog immunoassay itself, whose result tracks total testosterone far more closely than it tracks the dialyzable fraction Kacker 2013.
- Every error already in the total testosterone, including the platform bias that reverses direction across the range, since the calculation inherits it whole.
- The SHBG assay's own between-method spread, which propagates straight into the quotient Szybiak-Skora 2025.
Reference interval or decision threshold — which kind of number Free Testosterone is
It is a reference interval built on top of two other reference intervals, and its width shows it. A calculated value inherits the imprecision of the total, of the SHBG and of the albumin, so the confidence around a single free testosterone is wider than around any of its inputs. Nothing on the report says so.
Intervals are method-specific and equation-specific, which is unusual and important. A free testosterone from dialysis and one from a Vermeulen calculation are not interchangeable and must not be compared against the same printed range Fiers 2018 Hackbarth 2011.
In women the number is being asked to do diagnostic work. A diagnostic meta-analysis of androgen measurement in polycystic ovary syndrome found free testosterone the best-performing single androgen, which is precisely why the assay used to produce it matters more there than anywhere else Bizuneh 2024.
There is no outcome-derived threshold for free testosterone in men. Guideline decision-making runs on total testosterone plus symptoms, with free testosterone reserved for the case where SHBG is abnormal Livingston 2023. Treating a computed number as an optimum is inventing a threshold nobody validated.
How you would know your Free Testosterone was wrong — and when to redraw
Retest on the same clock as the total, because it is the same blood. Morning, fasted, rested. The free fraction has no separate biology and no separate timing; it is a ratio computed from the sample you already gave.
The interval belongs to the denominator. SHBG's own turnover and its response to insulin and thyroid state run over weeks to months, so a free testosterone repeated at two weeks after a body-composition change is measuring noise Szybiak-Skora 2025.
Conditions that must match: the same laboratory, the same assay for both parents, the same equation, and an albumin measured rather than assumed.
What would have to change for the second number to mean something:
- Free low but total normal? The explanation is in SHBG, and the confirming test is SHBG itself, not a repeat of this.
- Free normal but total low? Same test, opposite direction — a low binder. Check SHBG and check fasting insulin, since insulin resistance is the commonest cause of a suppressed SHBG in this readership Szybiak-Skora 2025.
- Free high in a woman with hirsutism? Confirm with a mass-spectrometry total testosterone and add 17-OH progesterone before accepting the calculated value Bizuneh 2024.
- Numbers that do not fit the picture? Ask whether the result came from an analog immunoassay. If it did, the disagreement is the expected finding, not a mystery Kacker 2013.
What Free Testosterone cannot tell you
It cannot be more accurate than its inputs, and it is usually less. Three measurements and one model, each with its own error, produce one number carrying all four.
It cannot resolve the bioavailable question it is often bought to answer. Albumin-bound testosterone dissociates fast enough to be available to tissue, so the biologically relevant pool is larger than the free fraction and smaller than the total, and no routine assay measures it directly Goldman 2017.
It cannot be compared to a value from another laboratory unless both the assay and the equation match Hackbarth 2011.
It cannot tell you anything the total and SHBG did not already say. When SHBG is normal, the calculated free testosterone is a restatement of the total in different units.
The wrong inference readers actually draw is that free testosterone is the truer number because it sounds more physiological. On most panels it is the least directly measured value on the report — often three inferences deep — and where SHBG is normal it adds nothing at all Fiers 2018 Livingston 2023.
Sources read for these sections
- Kacker R, et al. Free testosterone by direct and calculated measurement versus equilibrium dialysis in a clinical population. Aging Male 2013 · PMID 24090209
- Fiers T, et al. Reassessing Free-Testosterone Calculation by Liquid Chromatography-Tandem Mass Spectrometry Direct Equilibrium Dialysis. Journal of Clinical Endocrinology and Metabolism 2018 · PMID 29618085
- Hackbarth JS, et al. Accuracy of calculated free testosterone differs between equations and depends on gender and SHBG concentration. Steroids 2011 · PMID 20816687
- Goldman AL, et al. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocrine Reviews 2017 · PMID 28673039
- Dalmiglio C, et al. Analytical performance of free testosterone calculated by direct immunoluminometric method compared with the Vermeulen equation: results from a clinical series. Hormones 2024 · PMID 38177558
- Livingston M, et al. Adult Male Hypogonadism: A Laboratory Medicine Perspective on Its Diagnosis and Management. Diagnostics (Basel) 2023 · PMID 38132234
- Szybiak-Skora W, et al. New Insights in the Diagnostic Potential of Sex Hormone-Binding Globulin (SHBG)-Clinical Approach. Biomedicines 2025 · PMID 40427034
- Bizuneh AD, et al. Evaluating the diagnostic accuracy of androgen measurement in polycystic ovary syndrome: a systematic review and diagnostic meta-analysis to inform evidence-based guidelines. Human Reproduction Update 2024 · PMID 39305127
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
Calculated vs measured. Most 'free testosterone' results are calculated from total T and SHBG, so the result inherits every error in both. Direct analog immunoassays are worse still and are not recommended. Equilibrium dialysis is the reference method. If the number matters, ask which was used. - Read it with its partner
Insist on equilibrium dialysis or LC/MS-calculated free T. Direct analog assays routinely misclassify people — Marek offers the gold-standard version. Draw it alongside: Total Testosterone, SHBG (Sex Hormone-Binding Globulin), Comprehensive Metabolic Panel (CMP). - Work out which direction is yours
If it's high — Exogenous androgens, or low SHBG from insulin resistance, obesity, or hypothyroidism.
If it's low — The classic 'normal total T but symptomatic' presentation — fatigue, low libido, poor recovery, low mood. - Fix it in this order
Nutrition. Since free T is largely governed by SHBG: avoid chronic severe caloric restriction and very-low-fat diets (both raise SHBG). Adequate carbohydrate and energy availability keep SHBG in a healthy mid-range.
Lifestyle. Improve insulin sensitivity to modulate SHBG; resistance training; sleep. Treat hyperthyroidism if present.
Supplements. Boron has small-trial evidence for lowering SHBG and raising free T. Correct vitamin D and zinc. Magnesium may modestly reduce SHBG binding affinity.
Hormones. If total T is adequate but free T is low because SHBG is high, raising the T dose isn't always the answer — addressing the SHBG driver often is. Discuss with your provider.
Compounds. No peptide raises free T directly; the levers are total T and SHBG.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
With every total T retest — they must be interpreted together. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Goldman AL et al., Endocr Rev 2017 — critique of free T assays. Endocrine Society CPG recommends equilibrium dialysis or calculated free T.
This page can tell you what could have made your Free Testosterone wrong. It cannot tell you whether it did.
Everything above is free and stays free — the assay, what changes the reading rather than the blood, the retest window and the sources. What no page can do is look at your draw: which laboratory ran it, at what hour, what you were taking that week, and what else was flagged beside it. Every one of those changes the answer, and none of them is on any page. Bringing a real result to people who know that list is what the members' area is for.
Bring your result — $10/mo →🩸 Test your Free Testosterone
Order directly through Marek Diagnostics — no doctor's visit needed, drawn at any Quest location in the US. Code CAMERON applies 10% off automatically.
Order this test — 10% off → Browse all 103 markers →What Free Testosterone is usually tested alongside
One marker is a data point. These panels add the markers that make Free Testosterone interpretable, name why each is on the list, and load the set into your cart at 10% off.
includes this + 10 more markers · built for men — Fatigue, low libido, poor recovery, mood or body composition changes — and you're considering TRT. Read this before you start.
includes this + 10 more markers · built for men — You've decided to start testosterone therapy. Draw this before your first injection.
includes this + 9 more markers · built for women — Irregular or absent periods, acne, unwanted hair growth, scalp thinning, difficulty losing weight, or you've been told 'probably PCOS' without a workup.
includes this + 10 more markers · built for women — Anywhere from late 30s onward with cycle changes, night sweats, sleep disruption, mood shifts, joint aches, brain fog, or a libido that fell off a cliff.
What people use Free Testosterone to decide
Nobody orders a test for its own sake. Free Testosterone is on the test list for these pathways — each one links to what the pathway claims, and what its test list is read for before you spend anything on it.
Muscle loss on an aggressive deficit is invisible on the scale and obvious in IGF-1 and testosterone, both of which fall in a sustained deficit. Worth a baseline before you start and a recheck at eight weeks — this is the panel built specifically for people running a GLP-1.
Total testosterone alone tells you very little. Free T is the bioavailable fraction, SHBG explains why it might be low with a normal total, and LH/FSH tell you whether the problem is the testes or the pituitary — which decides the entire treatment.
LH and FSH decide the entire treatment. Low testosterone with HIGH LH means the testes have failed and no upstream stimulus will help; low testosterone with LOW LH is a pituitary signal problem, and that is the one this pathway fixes.
The most-missed pattern in men's health: a normal total testosterone with high SHBG produces every symptom of deficiency and a lab report that reads fine. Insulin resistance and thyroid both move SHBG, so treat the cause.
Free Testosterone is also on the test list for these, where it narrows the picture rather than settling it:
What moves your Free Testosterone
1 compound and 4 supplements in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 278 compounds & 371 supplements →
Would you feel it? Symptoms Free Testosterone helps explain
People search for how they feel, not for a marker. These are the complaints where this one is worth checking, and whether it is first-line or a follow-up.
Why your Free Testosterone might be wrong
Most abnormal results are interference, not disease. Check these before you change anything. Each says whether the number is wrong (repeat it), badly timed (redraw it), or real with a cause.
Most 'free testosterone' results are calculated from total T and SHBG, so the result inherits every error in both. Direct analog immunoassays are worse still and are not recommended.
Equilibrium dialysis is the reference method. If the number matters, ask which was used.
Follows total testosterone's morning peak, so the same timing rules apply.
Draw before 10am, consistently.
Because free T is derived from SHBG, thyroid hormone, estrogen, insulin resistance and liver disease all shift free T without the testicles doing anything differently.
Read SHBG alongside it, always.
What Free Testosterone means in combination
One marker tells you a little; combinations tell you the story. These are the named patterns this one takes part in.
High SHBG is binding up your testosterone so little is bioavailable. The total number looks fine and everyone tells you you're normal.
Investigate why SHBG is high: aggressive dieting, very-low-fat intake, hyperthyroidism, liver disease, aging, anticonvulsants. Often the fix is eating more and adding dietary fat rather than adding testosterone.
What to test next
These put Free Testosterone in context — each with its own full breakdown.
Frequently asked questions
Assay dependent — direct/analog ~46–224 pg/mL; equilibrium dialysis is the gold standard. Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Upper third of your assay's range. Prefer equilibrium dialysis or calculated free T — direct analog assays are widely criticized as inaccurate.
Exogenous androgens, or low SHBG from insulin resistance, obesity, or hypothyroidism.
The classic 'normal total T but symptomatic' presentation — fatigue, low libido, poor recovery, low mood.
You can order Free Testosterone directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.
Where this goes next
This page is the free framework, and it splits by sex. The protocol itself — the dosing, the order to correct things in, the week-by-week schedule and what to retest — is a lesson inside Skool.