Free Testosterone Index

SHBG is a glycoprotein that transports testosterone and estradiol in the blood. SHBG is synthesised in the liver and has a high binding affinity with 17-hydroxysteroid hormones.

Less than 2% of biologically active steroids are free in the circulation, the remainder being bound mainly to SHBG and albumin.

The concentration of SHBG in plasma is regulated by, among other things, the androgen/estrogen balance, thyroid hormones, insulin and dietary factors.
SHBG production is stimulated by oestradiol and inhibited by testosterone. As a result, SHBG concentrations are higher in women than in men.

Pregnant women have significantly higher serum SHBG levels due to their increased oestrogen production.

Plasma SHBG levels are influenced by a number of medical conditions, with elevated levels observed in hyperthyroidism, hypogonadism, androgen insensitivity and liver cirrhosis in men. Low levels are seen in myxedema, hyperprolactinaemia and excessive androgen activity syndromes.

Measurement of SHBG is useful in the assessment of mild disorders of androgen metabolism and can identify women with hirsutism who are more likely to respond to oestrogen therapy.

The ratio of testosterone to SHBG is also known as the Free Androgen Index (FAI) or the Free Testosterone Index (FTI). This ratio correlates closely with measured and calculated values of free testosterone and can be used to distinguish subjects with excessive androgen activity from normal individuals.