Free Androgen Index (FAI) Calculator

Determines the FAI ratio used in screening for abnormal androgen status in humans.

Refer to the text below the form for more information on the free androgen index and its clinical usage.


The free androgen index ratio between the total testosterone level and the sex hormone binding globulin (SHBG) helps screen for abnormal androgen status.

The FAI is intended to estimate the free testosterone level, the form in which it is able to bind to tissue receptors and exert its hormonal effect.


FAI = Total Testosterone (nmol/ L)) x 100 / Sex Hormone-Binding Globulin (SHBG) (nmol/ L)


Total testosterone:
Sex hormone-binding globulin (SHBG):
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


 

About the FAI

The testosterone in blood can be found under three forms: tightly bound to SHBG, weakly bound to albumin and only a small percentage (<3% in males and <0.7% in females) unbound. This third form is the free testosterone, the only form capable of binding to tissue receptors to exert its effects, hence why it makes for the best marker of androgen status.

FAI = Total Testosterone (nmol/ L)) x 100 / Sex Hormone-Binding Globulin (SHBG) (nmol/ L)

FAI was found in statistical analysis to be a poor predictor of bioavailable testosterone. FAI should not be used in isolation and is often accompanied by other measured or estimated parameters such as gonadotropin levels.

Because SHBG is present in such large excess in women (10–100:1), free testosterone concentrations are driven primarily by SHBG abundance. In addition, testosterone excess in women lowers SHBG concentrations, which raises the free testosterone concentration and contributes to the strong correlation of 1/SHBG with free testosterone.

Typical FAI healthy values range from 30 to 150 in adult men and 7 to 10 in adult women. In men, values below 30 are cause for concern and could contribute to erectile dysfunction whilst in women, higher values are cause for concern and could contribute to polycystic ovary syndrome and hirsutism.

 

References

Fraser, Marianne; Farrokh Sohrabi. "Free Androgen Index". Online Medical Encyclopedia. University of Rochester Medical Center.

Morris PD, Malkin CJ, Channer KS, Jones TH "A mathematical comparison of techniques to predict biologically available testosterone in a cohort of 1072 men" (PDF). Eur. J. Endocrinol. 2004; 151 (2): 241–9.

Dunn JF, Nisula BC, Rodbard D. Transport of steroid hormones: binding of 21 endogenous steroids to both testosterone-binding globulin and corticosteroid-binding globulin in human plasma. J Clin Endocrinol Metab. 1981; 53(1):58–68.


Specialty: Endocrinology

System: Endocrine

No. Of Variables: 2

Abbreviation: FAI

Article By: Denise Nedea

Published On: April 10, 2020

Last Checked: April 10, 2020

Next Review: April 10, 2025