Free & Bioavailable Testosterone Calculator
Estimates these specific testosterone concentrations based on the Vermeulen formulas.
Refer to the text below the tool for more information about the formulas employed and about normal ranges for free and bioavailable testosterone.
The sole measurement of total serum testosterone does not reflect the action of bioactive testosterone in the human body, hence why the determination of free and albumin bound testosterone is often employed and the results compared to established normal ranges.
Free Testosterone (mol/L) = [-b + sqrt (b2 +4a x T)] / 2a
Where:
a = kat + kt + (kat x kt) x (SHBG + Albumin – T)
b = 1 + kt x SHBG + kat x Albumin – (kat + kt) x T
kat = 3.6 x 104 L/mol; kt = 10 x 108 L/mol
T = testosterone in nmol/L; SHBG – in nmol/L; Albumin – in mg/dL;
Free Testosterone from mol/L to nmol/L, multiply by: 2.88 x 10-13 x 1.6 x 0.003448
Bioavailable Testosterone (nmol/L) = Free Testosterone in nmol/L x (1 + c)
Where:
c = 3.6 x 104 x albumin in g/L /69000
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Determining Free and Bioavailable Testosterone
The sole measurement of total serum testosterone does not reflect the action of bioactive testosterone in the human body, hence why the determination of free and albumin bound testosterone is often employed and the results compared to established normal ranges.
Testosterone and dihydrotestosterone (DHT) circulate in plasma in three ways:
- Unbound – free portion, usually between 2 and 3%;
- Strongly bound to specific plasma proteins, i.e. sex hormone-binding globulin (SHBG);
- Weakly bound to nonspecific plasma proteins, i.e. albumin.
The Vermeulen equation for calculated free testosterone is simplified below:
Free Testosterone (mol/L) = [-b + sqrt (b2 +4a x T)] / 2a
Where:
a = kat + kt + (kat x kt) x (SHBG + Albumin – T)
b = 1 + kt x SHBG + kat x Albumin – (kat + kt) x T
kat = 3.6 x 104 L/mol; kt = 10 x 108 L/mol
T = testosterone in nmol/L; SHBG – in nmol/L; Albumin – in mg/dL;
Free Testosterone from mol/L to nmol/L, multiply by: 2.88 x 10-13 x 1.6 x 0.003448
As the SHBG-bound fraction is biologically inactive – the free fraction and the albumin bound fraction are considered the bioavailable testosterone.
Bioavailable Testosterone (nmol/L) = Free Testosterone in nmol/L x (1 + c)
Where:
c = 3.6 x 104 x albumin in g/L /69000
Where the albumin is pre-set at 43 g/L (4.3 g/dL) – the c value becomes 22.43. If serum albumin is known, then adjust the value accordingly.
Normal Testosterone Ranges
Unbound and bioavailable testosterone adult normal ranges are summarized in the tables below:
Free Testosterone (ng/dL)
| Age | Adult male | Adult female |
| 20 – <25 | 5.25-20.7 | 0.06-1.08 |
| 25 – <30 | 5.05-19.8 | 0.06-1.06 |
| 30 – <35 | 4.85-19.0 | 0.06-1.03 |
| 35 – <40 | 4.65-18.1 | 0.06-1.00 |
| 40 – <45 | 4.46-17.1 | 0.06-0.98 |
| 45 – <50 | 4.26-16.4 | 0.06-0.95 |
| 50 – <55 | 4.06-15.6 | 0.06-0.92 |
| 55 – <60 | 3.87-14.7 | 0.06-0.90 |
| 60 – <65 | 3.67-13.9 | 0.06-0.87 |
| 65 – <70 | 3.47-13.0 | 0.06-0.84 |
| 70 – <75 | 3.28-12.2 | 0.06-0.82 |
| 75 – <80 | 3.08-11.3 | 0.06-0.79 |
| 80 – <85 | 2.88-10.5 | 0.06-0.76 |
| 85 – <90 | 2.69-9.61 | 0.06-0.73 |
Bioavailable Testosterone (ng/dL)
| Age | Adult male |
| 20 – <30 | 83-257 |
| 30 – <40 | 72-235 |
| 40 – <50 | 61-213 |
| 50 – <60 | 50-190 |
| 60 – <70 | 40-168 |
Bioavailable testosterone for adult females between 20 and 50 years depends on whether oral estrogen therapy is used:
- If on oral estrogen: 0.8 – 4.0 ng/dL
- If not on oral estrogen: 0.8 – 10.0 ng/dL
Abnormally decreased testosterone values, met in primary testicular failure, may be caused by:
- Genetic abnormalities (Klinefelter syndrome, XXY males);
- Developmental causes (eg, testicular maldescent);
- Testicular torsion or ischemia;
- Autoimmune polyglandular endocrine failure;
- Infections.
Abnormally increased testosterone values may be caused by testicular or adrenal tumors or androgen abuse when levels exceed the upper limit of normal by more than 50%.
In female patients, decreased testosterone levels may indicate primary or secondary ovarian failure, whilst increased levels may indicate congenital adrenal hyperplasia, ovarian or adrenal neoplasms or polycystic ovarian syndrome.
Some androgen and steroid medication may decrease testosterone concentrations whilst barbiturates, anticonvulsants or estrogen therapies may increase them.
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; 84(10):3666-3672.
de Ronde W, van der Schouw YT, Pols HA, et al. Calculation of bioavailable and free testosterone in men: a comparison of 5 published algorithms. Clin Chem. 2006; 52(9):1777-1784.
Manni A, Pardridge WM, Cefalu W, et al. Bioavailability of albumin-bound testosterone. J Clin Endocrinol Metab. 1985; 61:705.
Chung MC, Gombar S, Shi RZ. Implementation of Automated Calculation of Free and Bioavailable Testosterone in Epic Beaker Laboratory Information System. J Pathol Inform. 2017; 8:28.
Tsai EC, Matsumoto AM, Fujimoto WY, Boyko EJ. Association of bioavailable, free, and total testosterone with insulin resistance: influence of sex hormone-binding globulin and body fat. Diabetes Care. 2004; 27(4):861-868.
Specialty: Endocrinology
System: Endocrine
Article By: Denise Nedea
Published On: August 28, 2020 · 12:00 AM
Last Checked: August 28, 2020
Next Review: August 28, 2025