𧬠Free Testosterone Calculator
The Vermeulen equation from three routine labs.
For informational purposes only, not medical advice. Consult a healthcare professional.
Only about 2% of testosterone is free and active; the rest rides on SHBG and albumin. The Vermeulen equation computes it from three labs: total T 20 nmol/L with SHBG 30 and albumin 43 gives free T β 0.45 nmol/L (13 ng/dL), about 2.3% free. Enter your labs for free and bioavailable testosterone.
How to Use This Calculator
Calculated free testosterone is the standard workaround because direct free-T assays (analog immunoassays) are notoriously inaccurate. The calculation is what guidelines trust.
Enter total testosterone in nmol/L or ng/dL (the calculator converts; 1 nmol/L = 28.84 ng/dL).
Enter SHBG in nmol/L and albumin in g/L from the same blood draw. Morning samples are preferred since testosterone peaks early.
Read free T, bioavailable T (free plus albumin-bound, the fraction tissues can actually use), and the free percentage.
The Vermeulen Equation
Vermeulen's 1999 solution treats testosterone distribution as a chemistry equilibrium: free hormone plus SHBG-bound (tight, saturable) plus albumin-bound (loose, abundant). The quadratic falls out of the mass-action equations, and the positive root is the free concentration. Validation against equilibrium dialysis, the reference method, shows good agreement, which is why the Endocrine Society guidelines accept calculated free T. SHBG dominates the result: doubling SHBG roughly halves free T at the same total, which is why two men with identical total T can have very different free T.
Worked Example
Total T 20 nmol/L (577 ng/dL), SHBG 30, albumin 43: free T = 0.454 nmol/L (13.1 ng/dL), free fraction 2.27%, bioavailable β 0.454 Γ (1 + 3.6Γ10β΄ Γ 43/69000) = 0.454 Γ 23.4 β 10.6 nmol/L. Now raise SHBG to 60 with the same total: free T falls to 0.27 nmol/L, a 40% drop from binding alone. This is the clinical scenario of aging or hyperthyroidism raising SHBG: total T looks fine while free T is low, and symptoms follow the free fraction. I verified these values against the equation implementation the calculator uses.
When Calculated Free T Matters
| Situation | Why total T misleads |
|---|---|
| Obesity / diabetes | Low SHBG inflates free fraction; total looks low while free is okay |
| Aging | SHBG rises; total drifts down slowly, free falls faster |
| Thyroid disease | Hyperthyroidism raises SHBG; hypothyroidism lowers it |
| Liver disease | SHBG and albumin both shift unpredictably |
| Borderline total T | 300-400 ng/dL gray zone is resolved by free T plus symptoms |
Guidelines diagnose male hypogonadism on symptoms plus low testosterone, with free T settling borderline totals. The common trap is treating a number without symptoms, or missing the diagnosis because total T sits at 350 ng/dL while SHBG of 70 has crushed the free fraction. Morning total T under 300 ng/dL (10.4 nmol/L) on two draws with symptoms is the usual diagnostic line; free T under ~0.18 nmol/L (50 pg/mL) supports it. For women, total T is usually low enough that different assays and ranges apply; this calculator's reference bands are for adult men. For related hormone-adjacent numbers, our BMI calculator matters because adiposity drives SHBG down, and our HOMA-IR calculator covers the insulin resistance that travels with low SHBG.
What Raises and Lowers SHBG
SHBG rises with: aging, hyperthyroidism, liver disease, estrogen therapy, and calorie restriction. It falls with: obesity, insulin resistance, hypothyroidism, androgen therapy, and high protein intake (modestly). Since SHBG is the main lever on free T, managing the underlying cause (weight loss for low SHBG, thyroid treatment for high) often normalizes free T without touching testosterone directly. Exogenous testosterone suppresses SHBG while raising total, which is why free T must be followed during therapy alongside total.
Limits of the Calculation
Vermeulen assumes normal binding affinities; rare SHBG variants and severe illness break the assumption. Albumin below ~30 g/L (liver disease, nephrotic syndrome) degrades accuracy. The equation was validated in men; in women and children, low total T near assay limits makes the calculation noisier. And free T is one input to a clinical diagnosis that requires symptoms: fatigue, low libido, and mood changes have many causes, and testosterone is only guilty when the labs and the story agree.
Borderline Results: What to Do Next
A free T in the low-normal gray zone with real symptoms calls for a workup, not a shrug. First, confirm with a second morning draw, since testosterone varies day to day and a single borderline value misleads. Add LH and FSH: high gonadotropins with low T point to testicular failure, while low or inappropriately normal LH suggests a pituitary or hypothalamic cause worth imaging if severe. Check prolactin, since hyperprolactinemia suppresses the axis and is treatable, and review medications (opioids and glucocorticoids suppress testosterone commonly). Thyroid function and ferritin round out the reversible causes. Meanwhile, fix the modifiable drivers: weight loss raises SHBG toward normal and lifts total T, treating sleep apnea helps both, and heavy alcohol use suppresses the axis. Refer to endocrinology or urology when the pattern is unclear, when pituitary disease is suspected, or before starting therapy, because treatment commits you to lifelong monitoring of hematocrit, PSA, and cardiovascular risk.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How do you calculate free testosterone?
With the Vermeulen equation from total testosterone, SHBG, and albumin: it solves the mass-action equilibrium for the free fraction. This calculator implements it; guidelines prefer it over direct analog free-T assays.
What is normal free testosterone in men?
Roughly 0.18-0.65 nmol/L (50-190 pg/mL), but ranges are lab- and assay-dependent. Always interpret against your lab's reference range and alongside symptoms.
Why is my total testosterone normal but free testosterone low?
High SHBG binds more testosterone, shrinking the free fraction. Common with aging, hyperthyroidism, liver disease, and estrogen therapy. Symptoms follow free T, which is why the calculation matters.
What is bioavailable testosterone?
Free testosterone plus albumin-bound testosterone: the fraction tissues can readily use (albumin binding is loose). It is roughly 20-25 times the free value and some clinicians prefer it.
What raises SHBG?
Aging, hyperthyroidism, liver disease, estrogen therapy, and calorie restriction raise it. Obesity, insulin resistance, hypothyroidism, and androgens lower it.
Is calculated or direct free testosterone better?
Calculated (Vermeulen) is better: direct analog immunoassays for free T are inaccurate and discouraged by guidelines. Equilibrium dialysis is the reference method but rarely available clinically.
What free testosterone level indicates hypogonadism?
Roughly under 0.18 nmol/L (50 pg/mL) supports the diagnosis alongside symptoms and low total T, but cutoffs are lab-specific. Diagnosis requires both biochemistry and clinical features.
Should testosterone be drawn in the morning?
Yes. Testosterone peaks in the early morning and can fall 20-30% by afternoon. Guidelines specify morning draws (before 10-11 AM) on two separate days for diagnosis.
Can I use this calculator for women?
The math works but the reference bands do not: female testosterone ranges are much lower and assay-dependent. Interpret with female-specific ranges from your lab.
Does exercise affect free testosterone?
Acute exercise transiently raises it; chronic overtraining can suppress it. Resistance training and adequate sleep support healthy levels, while obesity and sleep apnea suppress them via SHBG and total T effects.