What Is Testosterone?
Testosterone is a steroid hormone present in both sexes, though it predominates in males. In men, the testes produce roughly 95% of circulating testosterone through Leydig cells, with the remaining 5% sourced from the adrenal cortex. Women produce smaller amounts in the ovaries and adrenal tissue. Beyond reproduction, testosterone drives skeletal muscle growth, bone density, red blood cell production, and cognitive function across the lifespan.
During male puberty, testosterone orchestrates the development of the penis, scrotum, prostate, and seminal vesicles. It also triggers secondary sexual characteristics: deepened voice, facial and body hair growth, and increased muscle mass. In women, testosterone supports bone strength, mood stability, and sexual function, though excess levels can trigger virilization (facial hair, voice deepening).
Testosterone does not work alone—it binds to transport proteins in the blood, which determines how much is available to tissues. This binding distinction is why total testosterone can mislead clinicians and patients alike.
Calculating Free and Bioavailable Testosterone
The Vermeulen formula estimates free testosterone by solving for the unbound fraction based on protein binding kinetics. It requires your albumin concentration, SHBG level, and total testosterone measurement. The calculator then derives bioavailable testosterone (free plus albumin-bound hormone, which is weakly reversible and accessible to cells).
n = 1 + (36,000 × albumin ÷ 69,000)
a = n × 10⁹
b = n + (10⁹ × (SHBG − total testosterone) ÷ 10⁹)
c = −total testosterone ÷ 10⁹
free testosterone = (−b + √(b² − 4ac)) ÷ (2a) × 10⁹
bioavailable testosterone = n × free testosterone
free testosterone % = free testosterone ÷ total testosterone
bioavailable testosterone % = bioavailable testosterone ÷ total testosterone
albumin— Serum albumin concentration in g/dL; normal range 3.4–5.4 g/dL. A major transport protein binding testosterone weakly.SHBG— Sex hormone-binding globulin in nmol/L or ng/mL (unit conversion handled internally). Binds testosterone strongly and reduces bioavailability.total testosterone— Sum of free, albumin-bound, and SHBG-bound testosterone in ng/dL or pmol/L. The standard clinical measurement.free testosterone— Unbound hormone available for receptor binding, expressed in pg/mL or pmol/L. Typically 1–4% of total testosterone.bioavailable testosterone— Free hormone plus albumin-bound testosterone. More clinically relevant than total testosterone in some cases.
Free Versus Bioavailable Testosterone
Free testosterone is the unbound fraction—roughly 1–4% of total testosterone in males and even less in females. Only free hormone can cross cell membranes and activate androgen receptors. For this reason, it is the most biologically active form.
Bioavailable testosterone includes free hormone plus albumin-bound testosterone. Albumin binding is weak and reversible, so albumin-bound testosterone is readily released and available for tissue uptake. Together, free and bioavailable testosterone often correlate better with clinical symptoms than total testosterone alone, especially when SHBG fluctuates due to age, liver disease, or hyperthyroidism.
A man with high SHBG (e.g., from thyroid disease) may have normal or high total testosterone but low free testosterone. Conversely, someone with low albumin (malnutrition, liver cirrhosis) may have reduced bioavailable testosterone despite adequate total levels. This is why clinical interpretation requires knowing the protein-binding breakdown.
When and Why SHBG Matters
SHBG is the primary regulator of testosterone availability. This globulin binds testosterone with high affinity, essentially sequestering it from cells. Higher SHBG concentrations reduce free testosterone; lower SHBG increases it.
SHBG production responds to several signals: estrogen (contraceptive pills, hormone replacement therapy, obesity in men) increases SHBG; testosterone, insulin, and thyroid hormone decrease it. Conditions raising SHBG include hyperthyroidism, cirrhosis, and HIV infection. Insulin resistance and type 2 diabetes lower SHBG. Aging typically reduces SHBG in men, paradoxically allowing a greater fraction of declining testosterone to remain free.
Race and genetic background also influence SHBG set-points. Men of African descent tend to have lower SHBG and higher free testosterone relative to total testosterone compared to men of European ancestry. These population differences complicate interpretation of reference ranges, making individual hormone profiles more informative than population averages.
Key Considerations When Using This Calculator
Accurate interpretation of testosterone fractions depends on understanding the calculator's strengths and limitations.
- Timing of Blood Draw — Testosterone peaks in early morning (6–8 a.m.) and drops by evening—sometimes by 25–30%. Always draw blood before 10 a.m. for reproducibility. Women should test in the follicular phase (days 1–5 of the menstrual cycle) when hormone levels stabilize, avoiding the mid-cycle surge and luteal fluctuations.
- SHBG Sensitivity — The calculator is most accurate when SHBG and albumin are stable. Acute illness, medications (phenytoin, barbiturates), or recent steroid use can distort SHBG. Renal disease and nephrotic syndrome may lower albumin dramatically. If these factors apply, discuss results with your clinician before drawing conclusions.
- Total Testosterone Measurement Method — Clinical labs use either immunoassay (fast, prone to cross-reactivity at low levels) or LC-MS (slower, more specific). At testosterone levels below 100 ng/dL, immunoassay accuracy suffers significantly. Request LC-MS if you have symptoms of deficiency but total testosterone tests as borderline or low-normal.
- Reference Ranges Vary — 'Normal' testosterone ranges differ by age, sex, assay platform, and population. A 65-year-old man with 400 ng/dL total testosterone may have a free testosterone level that feels low by younger standards. Symptoms, not numbers alone, drive treatment decisions. Combine this calculator's output with clinical context and your doctor's judgment.