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Free androgen index calculator

Calculate the free androgen index from total testosterone and SHBG, with automatic unit conversion and reference ranges for women and men.

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Sex

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nmol/L

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Results

Free androgen index (FAI)

2.8

Within the usual female range
Total testosterone in nmol/L
1.39 nmol/L
Total testosterone in ng/dL
40 ng/dL
Testosterone to SHBG molar ratio
0.028

This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.

What the free androgen index measures

Most testosterone in the blood is bound to proteins: tightly to sex hormone-binding globulin (SHBG) and loosely to albumin. Only the unbound fraction and the loosely bound part act on tissues. The free androgen index (FAI) is a simple way to estimate how much testosterone is available, by relating total testosterone to SHBG.

The formula

FAI = 100 × total testosterone (nmol/L) ÷ SHBG (nmol/L)

Laboratories in the United States often report testosterone in ng/dL: multiply by 0.0347 to convert to nmol/L (40 ng/dL = 1.39 nmol/L). The calculator does the conversion when you pick the unit.

Example

A woman with a total testosterone of 40 ng/dL (1.39 nmol/L) and an SHBG of 50 nmol/L has a FAI of 100 × 1.39 ÷ 50 = 2.8.

Group Usual FAI
Women of reproductive age about 0.5 to 5
Women with polycystic ovary syndrome often above 5
Adult men about 30 to 150

Reference ranges depend on the laboratory and the testosterone assay; use the ones printed on your report when they are available.

When it is used

  • Women: to look for hyperandrogenism, especially in polycystic ovary syndrome (PCOS), where the FAI is one of the most sensitive blood markers. The Rotterdam criteria accept biochemical or clinical signs of excess androgens.
  • Men: the FAI is less reliable, because men’s SHBG is lower and their testosterone is higher. Calculated free testosterone (Vermeulen formula) or measured free testosterone is preferred for diagnosing low testosterone.

What changes SHBG

SHBG rises with estrogen, oral contraceptives, hyperthyroidism, liver disease and aging. It falls with obesity, insulin resistance, hypothyroidism and androgen use. A low SHBG raises the FAI even when total testosterone is normal, which is why insulin resistance and PCOS so often go together.

Testosterone should be measured in the morning, ideally with a mass spectrometry assay for women, whose levels are low.

Frequently asked questions

What is a normal free androgen index for a woman?

It is usually below about 5, often between 0.5 and 5. Higher values suggest an excess of available testosterone, as in polycystic ovary syndrome.

How do I convert testosterone from ng/dL to nmol/L?

Multiply by 0.0347. For example, 40 ng/dL equals about 1.39 nmol/L. From ng/mL, multiply by 3.467.

Why is the FAI less useful in men?

Men have lower SHBG and much higher testosterone, so the ratio no longer tracks free testosterone well. Calculated free testosterone is preferred.

Do birth control pills change the FAI?

Yes. Combined pills raise SHBG and lower ovarian androgen production, so the FAI falls. Results taken on the pill do not reflect your usual androgen status.

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Results are estimates provided for information purposes. They do not replace professional advice.

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