Free androgen index (FAI) (LAI)

The free androgen index (FAI) is not measured directly in the blood; it is calculated with a formula: total testosterone divided by SHBG and multiplied by 100. SHBG (sex hormone binding globulin) is a protein that binds most of the testosterone in the blood and keeps it from reaching the tissues — the free, unbound part is the biologically active one. That is why the amount of total testosterone alone is often not enough: if SHBG is low or high, the amount of biologically active testosterone can differ from what total testosterone shows. The FAI helps assess that ratio in a single number. The test is done by ordering the testosterone and SHBG tests together — the index is calculated from their results. It is most often used in women when an excess of androgens (hyperandrogenism) is suspected, for example when there are signs of polycystic ovary syndrome (PCOS).

• In women, when there are signs of a raised level of androgens — excessive hair growth on the face or body, acne, thinning hair on the scalp
• When polycystic ovary syndrome (PCOS) or another cause of hyperandrogenism is suspected
• When total testosterone is borderline or does not explain the symptoms — to assess the biologically active part of the testosterone
• In men, when a deficiency or an excess of testosterone is suspected and the aim is to clarify how SHBG affects the result
• To monitor the course of treatment when the hormonal situation changes (for example thyroid disorders, obesity, liver disease)

The FAI is not one universal figure. It is calculated from the ratio between total testosterone and SHBG, and the reference interval depends on the laboratory method, on sex and on age. You will find the exact interval on your own laboratory report, and it is worth going through it together with your doctor. One index value on its own does not name a cause: the result is assessed together with the other indicators and with a doctor.

A raised index

A value taken to be raised is usually linked to a raised total testosterone or a lowered SHBG — this can mean a larger amount of biologically active androgen. In women the most common cause is polycystic ovary syndrome (PCOS), in which SHBG tends to be lowered. Other causes are adrenal disorders, some ovarian changes and the use of steroid medicines.

• Polycystic ovary syndrome (PCOS) in women — it often lowers SHBG and at the same time raises testosterone
• A disorder of the adrenal gland (including congenital adrenal hyperplasia)
• Ovarian tumours or other ovarian changes in women
• Insulin resistance, type 2 diabetes or obesity — often linked to a lower SHBG
• Steroid or anabolic medicines — they can lower SHBG

Symptoms that may go with a raised index: in women — excessive hair growth on the face or body, acne, menstrual irregularities; and thinning hair on the scalp.

• Total testosterone and SHBG separately — so that it is clear which of the two produced the change in the index
• In women — an ovarian ultrasound scan and other hormone tests (LH, FSH), if PCOS is suspected
• Liver and thyroid function and the body mass index — they affect the amount of SHBG
• Discussing with your doctor whether further tests are needed to establish the cause

A value taken to be low is usually linked to a lowered total testosterone or a raised SHBG — this can mean a smaller amount of biologically active androgen. In men it may point to disorders of the testicles or of the pituitary gland, and a raised SHBG can be linked to liver disease or to hyperthyroidism.

• In men — disorders of the testicles, a disorder of the pituitary gland, chronic liver or kidney disease, which lower testosterone
• A raised SHBG because of liver disease, hyperthyroidism (an overactive thyroid) or certain eating disorders
• The use of medicines containing oestrogen (for example hormone therapy during the menopause or contraceptive pills) in women — this can raise SHBG
• Reduced production of sex hormones in men

• Total testosterone and SHBG separately — so that it is clear which of the two produced the change in the index
• In men — an assessment of pituitary and testicular function, if a testosterone deficiency is suspected
• A review of the medicines you take, together with your doctor — some medicines affect the amount of SHBG
• Liver and thyroid indicators, if SHBG is raised without a clear reason

Reading the result

Because the index is calculated from two results, a change in it is read alongside total testosterone and SHBG — those two show which of them produced the shift. A single result does not establish a diagnosis: with PCOS, for example, the diagnosis is made from several criteria together, not from the index alone, and that is discussed with a doctor. The same logic applies here as to reading blood test results in general — the number is compared with the interval printed on your own report and assessed together with the other indicators.

Related indicators: DHEA-S, testosterone, SHBG, LH, FSH and prolactin.