By Austin Witherow and Mason Calvert · Last updated · Sources
What the numbers mean
- Total testosterone is everything a lab measures: free, bound to SHBG, and bound to albumin.
- Free testosterone is the small part not bound to any protein.
- Bioavailable testosterone is free plus albumin-bound. Albumin holds testosterone loosely, so this part is counted as available to tissues. SHBG holds it tightly, so that part is not.
SHBG is what moves free testosterone most. Take the same total of 600 ng/dL. With SHBG at 20 nmol/L, free testosterone comes out at 166 pg/mL. At 60 nmol/L it is 86.2 pg/mL, 48.1% lower. Two people with the same total can have very different free levels.
How it's calculated
The calculator uses the mass-action equation from Vermeulen, Verdonck and Kaufman (1999). Total testosterone is split between free, SHBG-bound and albumin-bound, and each binding follows its association constant. Solving that balance gives a quadratic equation, and free testosterone is its positive root.
| Constant | Value | Source |
|---|---|---|
| Testosterone–SHBG association | 1 × 10⁹ L/mol | Sato et al. 2017 |
| Testosterone–albumin association | 3.6 × 10⁴ L/mol | Sato et al. 2017 |
| Albumin when left blank | 4.3 g/dL | Sato et al. 2017 |
| Albumin molar mass | 69,000 g/mol | Chung et al. 2017 |
| Testosterone molar mass | 288.4 g/mol | PubChem |
Units: 1 nmol/L of testosterone is 28.8 ng/dL. Free testosterone in pmol/L is nmol/L × 1,000, and in pg/mL it is nmol/L × 288.4. The example values above, 600 ng/dL, are 20.8 nmol/L. With SHBG 35 nmol/L and albumin 4.3 g/dL, that gives 125 pg/mL free (2.09%).
Limits
- It is an estimate. Free testosterone is measured directly by equilibrium dialysis. Vermeulen found the calculated value close to dialysis except in pregnancy, when estradiol takes up SHBG binding sites.
- Published SHBG constants vary, from 0.6 to 1.9 × 10⁹ L/mol (Carruthers et al. 2007). Calculators that use a different constant give different numbers from the same labs.
- Labs differ in assay and reference range, especially for SHBG.
- Pedplot gives no reference range and does not call a result low or normal.
- The calculator accepts SHBG up to 500 nmol/L. A larger number usually means a unit mix-up.
To see how injected testosterone esters rise and fall over time, a different question from a single lab draw, see how testosterone esters behave over time.
Questions
How does SHBG change free testosterone?
More SHBG binds more testosterone, leaving less free. In the example above, raising SHBG from 20 to 60 nmol/L at the same total lowers free testosterone by 48.1%. See what the numbers mean.
How do I convert testosterone from nmol/L to ng/dL?
Multiply nmol/L by 28.8. For example, 20.8 nmol/L is 600 ng/dL. The unit menu converts the field for you. The factor comes from testosterone's molar mass, 288.4 g/mol.
Is calculated free testosterone the same as a measured one?
No. Equilibrium dialysis measures it directly; this calculates it from total testosterone and SHBG. Vermeulen 1999 found the two almost identical outside pregnancy. See limits.
Does albumin matter?
A little. With the example values, albumin at 3.5 g/dL instead of 4.3 raises free testosterone from 125 to 140 pg/mL. Leave the field blank if your report has no albumin.
Sources
- Paper. 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;84:3666–72 (abstract)
- Paper. Sato et al. 2017, Hum Reprod Open, Methods: Vermeulen's constants, SHBG 1 × 10⁹ and albumin 3.6 × 10⁴, with albumin fixed at 43 g/L
- Paper. Chung et al. 2017, J Pathol Inform, Materials and Methods: the equation as run in a hospital lab system, with 69,000 g/mol albumin and 28.84 ng/dL per nmol/L
- Paper. Carruthers et al. 2007, The validity of androgen assays, Aging Male: published SHBG constants range from 0.6 to 1.9 × 10⁹ L/mol
- Computed (PubChem). PubChem, testosterone (CID 6013): molecular weight 288.4 g/mol
The model and its limits are on the methodology page.