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Free Testosterone Calculator (Vermeulen Method)
Calculate free and bioavailable testosterone using total testosterone, SHBG, and albumin. Includes age-adjusted references, SHBG interpretation, testosterone fractions visualization.
⚠️ Medical Disclaimer: Calculated free testosterone using the Vermeulen method is an estimate. Direct equilibrium dialysis is the gold standard. Results should be interpreted with clinical context by an endocrinologist.
Presets:
years
ng/dL
nmol/L
g/dL
Planning notes, formulas, and examples
About the Free Testosterone Calculator (Vermeulen Method)
The Free Testosterone Calculator estimates free and bioavailable testosterone from total testosterone, SHBG (sex hormone-binding globulin), and albumin using the Vermeulen mass-action equation. Only 1–3% of circulating testosterone is unbound ("free"), with ~40–60% bound to SHBG (tightly, biologically inactive) and ~40–50% loosely bound to albumin (bioavailable). Since most testosterone assays measure total testosterone, calculating the free fraction is essential for accurate clinical assessment.
Free testosterone is the most clinically relevant testosterone measurement because only the unbound fraction can enter cells and exert androgenic effects. Conditions that alter SHBG levels (obesity, diabetes, liver disease, thyroid disorders, aging, estrogen use) can make total testosterone misleading — a man with normal total T but high SHBG may actually have clinically low free testosterone. The Endocrine Society recommends calculated free testosterone when SHBG abnormalities are suspected.
The Vermeulen method uses mass-action equilibrium calculations with the known binding constants of testosterone to SHBG (Ka = 1.0 × 10⁹ L/mol) and albumin (Ka = 3.6 × 10⁴ L/mol). It has been validated against equilibrium dialysis (the gold standard) and is used by most online free testosterone calculators, including the Endocrine Society's calculator. It shows sex-specific and age-adjusted reference ranges for proper clinical interpretation.
When This Page Helps
Total testosterone can look reassuring even when SHBG is shifting the active fraction up or down. This calculator keeps the binding inputs and the calculated free and bioavailable values together, which makes it easier to interpret testosterone results in the context they were drawn and to compare one measurement with another over time.
How to Use the Inputs
- Select biological sex for appropriate reference ranges.
- Enter age for age-adjusted interpretation.
- Enter total testosterone with the correct unit (ng/dL, nmol/L, or ng/mL).
- Enter SHBG level (nmol/L or µg/dL).
- Enter albumin (default 4.3 g/dL if not measured).
- Review free testosterone, bioavailable fraction, and clinical interpretation.
Formula used
Vermeulen Mass-Action Equation:
Binding constants:
• Ka (T-SHBG) = 1.0 × 10⁹ L/mol
• Ka (T-Albumin) = 3.6 × 10⁴ L/mol
Free T is solved iteratively from:
Free T = Total T / (1 + Ka_SHBG × [SHBG_free] + Ka_Alb × [Albumin])
where SHBG_free = SHBG − SHBG-bound T
Bioavailable T = Free T + Albumin-bound T
Unit conversions:
• ng/dL → nmol/L: × 0.0347
• pg/mL → pmol/L: × 3.467Example Calculation
Result: Free T ≈ 100 pmol/L (10 pg/mL), ~2% of total — within normal range for age 45.
With total T of 500 ng/dL (17.4 nmol/L) and normal SHBG of 40 nmol/L, approximately 2% is free and ~50% is bioavailable. This is within the normal range for a 45-year-old male (reference: 65–175 pmol/L). If SHBG were elevated to 80 nmol/L with the same total T, free T would drop to ~55 pmol/L — potentially below the hypogonadal threshold despite "normal" total testosterone.
Tips & Best Practices
- If SHBG is not available, request it — calculating free T without SHBG is not possible with the Vermeulen method.
- Albumin of 4.3 g/dL is a reasonable default for most patients. Only adjust if the patient has known hypoalbuminemia (liver disease, nephrotic syndrome, malnutrition).
- Morning blood draws (7–10 AM) are recommended for testosterone — levels follow a circadian rhythm with peaks in early morning.
- Fasting is recommended before testosterone testing as postprandial states can acutely lower testosterone by 15–25%.
- In obesity, both total T and SHBG are low, but free T may be normal or only mildly reduced — always check free T in obese men before diagnosing hypogonadism.
- The Free Androgen Index (FAI = Total T / SHBG × 100) is sometimes used but is less accurate than the Vermeulen calculation, especially in men.
Clinical Use in Hypogonadism Diagnosis
Endocrine Society guidance recommends: 1) Measure total testosterone (morning, fasting) on two separate occasions to confirm low levels. 2) If total T is low-normal (200–350 ng/dL) or SHBG abnormalities are suspected, calculate free testosterone. 3) Hypogonadism diagnosis requires both low testosterone AND symptoms (fatigue, decreased libido, erectile dysfunction, decreased muscle mass, depressed mood). Treatment (TRT) is not recommended based on lab values alone.
PCOS and Female Androgen Assessment
In women, free testosterone is essential for evaluating androgen excess in PCOS (polycystic ovary syndrome). The Rotterdam criteria require 2 of 3: hyperandrogenism, oligo/anovulation, polycystic ovaries. Biochemical hyperandrogenism is best assessed by free testosterone (or free androgen index). Many women with PCOS have normal total testosterone but elevated free T due to low SHBG (driven by insulin resistance). Weight loss and metformin improve insulin sensitivity, raise SHBG, and lower free testosterone in PCOS.
Monitoring Testosterone Replacement
During testosterone replacement therapy (TRT), free testosterone monitoring ensures adequate dosing. Target: mid-normal range for age. Trough levels (before next injection/application) are most informative. SHBG may decrease during TRT (especially oral formulations), affecting the total-to-free ratio. Monitoring should include: free T (target), hematocrit (<54%), PSA (baseline and annual), and lipids. Overreplacement risks: polycythemia, sleep apnea worsening, prostate stimulation, and cardiovascular events (debated).
Sources & Methodology
Last updated:
Methodology
This page applies the Vermeulen mass-action method to estimate the free and albumin-bound fractions of testosterone from total testosterone, SHBG, and albumin. It then reports the calculated free and bioavailable fractions beside sex- and age-context notes so the result can be reviewed in the same frame as the measured binding proteins.
The output is a laboratory interpretation aid, not a diagnosis of hypogonadism or androgen excess by itself. Symptoms, assay quality, timing of the blood draw, repeat testing, medication use, and the broader endocrine picture still determine what the number means clinically.
Sources
- A critical evaluation of simple methods for the estimation of free testosterone in serum (The Journal of Clinical Endocrinology & Metabolism) — Original Vermeulen reference for calculated free testosterone.
- Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline (The Journal of Clinical Endocrinology & Metabolism) — Guideline context for when calculated free testosterone is useful in evaluation.
Frequently Asked Questions
Only free testosterone (1–3% of total) can cross cell membranes, bind androgen receptors, and exert biological effects. The ~60% bound to SHBG is functionally inactive. Total testosterone can be misleading: a man with 450 ng/dL total T and high SHBG (80 nmol/L) may have low free T and symptomatic hypogonadism, while a man with 350 ng/dL total T but low SHBG (20 nmol/L) may have adequate free T with no symptoms. The Endocrine Society recommends checking free testosterone when total T is borderline or SHBG abnormalities are suspected.
Free testosterone is completely unbound (~1–3% of total). Bioavailable testosterone includes free T plus albumin-bound T (~40–50% of total). Since albumin binding is weak (Ka = 3.6 × 10⁴ vs. SHBG Ka = 1.0 × 10⁹), albumin-bound T readily dissociates and is considered biologically available. Some clinicians prefer bioavailable T as a clinical indicator. The SHBG-bound fraction (~40–60%) is tightly bound and biologically inactive under normal conditions.
The Vermeulen calculation correlates well with equilibrium dialysis (r = 0.91–0.97 in validation studies) and is considered clinically acceptable for routine use. Equilibrium dialysis is the gold standard but is expensive, labor-intensive, and only available at reference laboratories. The main limitation of the Vermeulen method is at extreme SHBG levels (very low or very high) where the calculation may diverge from dialysis results. For most clinical scenarios, the calculated free T is sufficiently accurate for diagnosis and treatment decisions.
SHBG increases with: aging (1–2% per year after 40), hyperthyroidism, liver cirrhosis, estrogen use (oral contraceptives, hormone replacement), anticonvulsants (phenytoin, carbamazepine), HIV, and anorexia. High SHBG lowers free testosterone even with normal total T. In men, this is a common cause of symptomatic hypogonadism with "normal" total testosterone levels. Treatment of the underlying cause (e.g., correcting hyperthyroidism) often normalizes SHBG and restores free T levels.
SHBG decreases with: obesity (insulin directly suppresses SHBG production), type 2 diabetes / insulin resistance, hypothyroidism, nephrotic syndrome, exogenous androgen use (testosterone, anabolic steroids), growth hormone excess (acromegaly), and glucocorticoid excess. Low SHBG results in higher free testosterone relative to total T. In women, low SHBG contributes to androgen excess symptoms (hirsuitism, acne, alopecia) even with normal total testosterone — this is a key mechanism in PCOS.
Total testosterone decreases approximately 1–2% per year after age 30 in men, with free testosterone declining even faster (~2–3% per year) because SHBG increases with age. By age 70, total T has declined ~30% and free T ~50% compared to age 30. However, "normal aging" vs. pathological hypogonadism remains debated. The Endocrine Society uses age-specific reference ranges and recommends against treating based solely on age-related decline without symptoms. In women, testosterone declines after ovarian senescence but the clinical significance is less well-defined.
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