What does it measure?
The laboratory divides total testosterone by sex hormone binding globulin, or SHBG, using compatible units, then multiplies by 100. This estimates a relationship between a hormone and its carrier protein. It does not directly measure free testosterone. [1]
Before reading the calculation
- Inputs
- Calculated from total testosterone and SHBG measured in blood.
- Interpretation
- Used in selected androgen assessments, particularly in women; it is not a universal hormone score.
What can the result mean?
A higher index can occur when testosterone is higher or SHBG is lower. It may help evaluate androgen-related symptoms in women, such as excess facial hair. A lower index does not establish a hormone deficiency, and the calculation is not a reliable universal tool for men. [1]
Calculated from total testosterone and SHBG measured in blood.
- Calculation
- The laboratory divides total testosterone by sex hormone binding globulin, or SHBG, using compatible units, then multiplies by 100. This estimates a relationship between a hormone and its carrier protein. It does not directly measure free testosterone.
- Interpretation
- Used in selected androgen assessments, particularly in women; it is not a universal hormone score.
When is it useful, and how is it read?
It may help assess androgen-related symptoms in women within a suitable clinical setting. Interpret the two underlying results, cycle and medicine history, and SHBG extremes; the calculation should not be applied as a universal hormone score for men or women. [1]
What can affect it?
Extremely low or high SHBG can make the index misleading. Hormonal contraception, thyroid or liver conditions, and metabolic changes can alter SHBG. Interpretation depends on the quality of both underlying measurements and the clinical question.
Myth: «The index is a direct free-testosterone measurement»
It is calculated from total testosterone and SHBG. A change in the carrier protein can move the index even without a matching change in testosterone production. [1]
Questions you might have
Is this a direct measurement of free testosterone?
No. It is an index based on two other measurements. [1]
Why can the index change when total testosterone does not?
A change in SHBG changes the ratio, which is why both inputs need to be read together. [1]
What comes next?
Review the actual testosterone and SHBG results, not just the ratio. Your clinician may prefer another free-testosterone assessment when the calculation is unsuitable. A high FAI alone does not diagnose polycystic ovary syndrome.
Is this calculation reliable with my SHBG level and appropriate for my symptoms?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- York and Scarborough Teaching Hospitals NHS — Free Androgen Index
- Mayo Clinic Laboratories. Testosterone, Total, Bioavailable, and Free, Serum (TTFB).
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. 2018;103:1715–1744.
Books we consulted
References that informed our explanations of the test and its context.
- Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, editors. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
