Free & Bioavailable Testosterone Calculator

Free & Bioavailable Testosterone Calculator for Endocrinology. Report free testosterone in ng/dL or pg/mL and compare against a laboratory's own reference interval, since assay-derived SHBG and total testosterone values vary. Commonly cited adult male lower limits are approximately 6.5 ng/dL (65 pg/mL) for calculated free testosterone and around 130-150 ng/dL for bioavailable testosterone, though these are method-dependent. A value below the reference limit in a symptomatic man, confirmed on a second fasting morning sample, supports hypogonadism and prompts a workup for its cause. A calculated free testosterone that is normal despite a low total reassures against true androgen deficiency and usually reflects low SHBG.

How this calculator works

This tool applies the Vermeulen equation, which models testosterone's mass-action equilibrium across three pools: SHBG-bound, albumin-bound, and free. Inputs are total testosterone, SHBG, and albumin (albumin defaults to ~4.3 g/dL when unmeasured). Using an SHBG association constant of ~1 x 10^9 L/mol and an albumin constant of ~3.6 x 10^4 L/mol, it solves a quadratic to yield free testosterone (typically ~1-3% of total). Bioavailable testosterone is then computed as free plus the loosely albumin-bound fraction (free / [albumin-bound + free] ratio), representing the biologically accessible pool.

When to use this calculator

Use when a man's total testosterone is borderline (roughly 200-350 ng/dL) or when SHBG is expected to be abnormal, since total testosterone misclassifies these patients. SHBG is lowered by obesity, insulin resistance, type 2 diabetes, nephrotic syndrome, hypothyroidism, and glucocorticoids, and raised by aging, hyperthyroidism, HIV, cirrhosis, and estrogens. In these settings free/bioavailable testosterone better reflects androgen status. Do not apply it in pregnancy, where SHBG immunoassay overestimates true binding capacity and calculated values fall spuriously low, and it is not validated for monitoring exogenous testosterone therapy.

Inputs used

  • Total testosterone
  • SHBG
  • Albumin

Clinical interpretation

Report free testosterone in ng/dL or pg/mL and compare against a laboratory's own reference interval, since assay-derived SHBG and total testosterone values vary. Commonly cited adult male lower limits are approximately 6.5 ng/dL (65 pg/mL) for calculated free testosterone and around 130-150 ng/dL for bioavailable testosterone, though these are method-dependent. A value below the reference limit in a symptomatic man, confirmed on a second fasting morning sample, supports hypogonadism and prompts a workup for its cause. A calculated free testosterone that is normal despite a low total reassures against true androgen deficiency and usually reflects low SHBG.

Worked example

A 58-year-old man with total testosterone 320 ng/dL (11.1 nmol/L), SHBG 65 nmol/L (elevated), and albumin 4.3 g/dL. Despite a total that looks low-normal, high SHBG sequesters most of the hormone: calculated free testosterone is approximately 4.5 ng/dL (~0.156 nmol/L) and bioavailable testosterone about 108 ng/dL. Free testosterone below roughly 6.5 ng/dL (~65 pg/mL) supports biochemical hypogonadism, so this patient is androgen-deficient even though his total value alone would not flag it.

Limitations and safety notes

The output is only as accurate as its inputs: measure total testosterone by a reliable morning fasting assay (ideally LC-MS/MS) and SHBG by immunoassay, since analog direct free-testosterone immunoassays correlate poorly and should not substitute. The Vermeulen constants assume standard binding affinities and 1:1 SHBG stoichiometry; work questioning single-site SHBG binding suggests the equation can overestimate free testosterone at high SHBG. It is invalid in pregnancy and is not established in children, and reference cutoffs derive largely from Western adult male cohorts.

Frequently asked questions

Why calculate free testosterone instead of measuring it directly?

Equilibrium dialysis is the reference standard but is labor-intensive and not widely available, while direct analog free-testosterone immunoassays correlate poorly with dialysis. The calculated value from total testosterone, SHBG, and albumin tracks dialysis-measured free testosterone closely and is practical for routine use.

What is the difference between free and bioavailable testosterone?

Free testosterone is the unbound fraction (about 1-3% of total). Bioavailable testosterone adds the loosely albumin-bound portion, since that low-affinity binding readily dissociates and is tissue-accessible. Bioavailable therefore represents a larger biologically active pool than free alone.

Does albumin need to be entered?

If albumin is unavailable the calculator assumes a normal value near 4.3 g/dL, which is adequate for most patients. Entering a measured albumin matters in hypoalbuminemic states such as nephrotic syndrome or cirrhosis, where a low albumin meaningfully shifts the calculated free and bioavailable fractions.

When does the Vermeulen equation give misleading results?

In pregnancy, high estradiol occupies SHBG sites so immunoassay overestimates binding capacity and calculated free testosterone falls falsely low. Accuracy also degrades at extreme SHBG values, and the calculation cannot be used to interpret levels in men on exogenous testosterone.

References

  • Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999. PMID: 10523012.
  • Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. PMID: 29562364.
  • Yeap BB, Wu FCW. Clinical practice update on testosterone therapy for male hypogonadism: Contrasting perspectives to optimize care. Clin Endocrinol (Oxf). 2018. PMID: 30358898.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: May 24, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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