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.
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 analogue 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.
🩸 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 102 markers →What Free Testosterone is usually tested alongside
On its own, one marker is a data point. These panels include Free Testosterone plus the markers that make it interpretable — each names every test, why it is on the list, and loads the whole set into your cart in one click 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 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 237 compounds & 350 supplements →
Would you feel it? Symptoms Free Testosterone helps explain
People rarely search for a marker — they search for how they feel. These are the complaints where this marker is worth checking, and whether it is a first-line test or a follow-up once the obvious causes are ruled out.
Why your Free Testosterone might be wrong
Most abnormal results are interference, not disease. These are the things that measurably move this specific marker — check them before you change anything. Each one says whether the number is wrong and should be repeated, badly timed and should be redrawn, or real with a cause worth explaining.
Most 'free testosterone' results are calculated from total T and SHBG, so the result inherits every error in both. Direct analogue 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, oestrogen, 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
A single marker tells you a little. Combinations tell you the story — these are the named patterns this marker takes part in, and what each one points at.
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
Markers rarely answer alone. These are the ones that 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.