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Free androgen index (FAI) - description and reference ranges

What is it?

Androgens are hormones that produce male characteristics in a body, such as facial hair. Both women and men have androgents, but women usually in very small concentrations. The Free Androgen Index (FAI) gives an estimate of how much free testosterone is present in the blood. It is a calculation is recommended as a baseline investigation for suspected hyperandrogenism in females.

It is calculated as Free androgen index (FAI) = Total Testosterone/SHBG x 100% and is given as a percentage.

It is important that you collect blood for this test in the early morning, after a fast.

Reference ranges

If your indicative FAI is lower than the reference range for our laboratory:

Low FAI in women is generally not a cause for concern, it is the elevated levels of androgens that are normally investigated.

Low values in men may indicate testosterone deficiency, which may contribute to fatigue, erectile dysfunction, osteoporosis and loss of secondary sex characteristics.

You can discuss this result with your GP.

If your indicative FAI is higher than the reference range for our laboratory:

In women, raised androgens can be a concern in hirsutism or the polycystic ovary syndrome.

You can discuss this result with your GP.

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