Sex hormone-binding globulin (SHBG) is a glycoprotein that binds to sex hormones, specifically testosterone and estradiol. Other steroid hormones such as progesterone, cortisol, and other corticosteroids are bound by transcortin.

Transport of sex hormones

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These sex hormones circulate in the bloodstream, bound mostly to SHBG and to some degree bound to albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor. Thus bioavailability of sex hormones is influenced by the level of SHBG.

SHBG production

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SHBG is produced by the liver cells and released into the bloodstream. Other sites that produce SHBG are the brain, uterus, and placenta. In addition SHBG is produced by the testis; testis-produced SHBG is also called androgen-binding protein. The gene for SHBG is located on chromosome 17.

Control

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SHBG levels appear to be controlled by a delicate balance of enhancing and inhibiting factors. Its level is decreased by high levels of insulin and insulin-like growth factor-I (IGF-I). Also, high androgen levels decrease SHBG, while high estrogen and thyroxine levels increase it.

Conditions with high or low levels

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Conditions with low SHBG include polycystic ovary syndrome, diabetes, and hypothyroidism. Conditions with high SHBG include pregnancy, hyperthyroidism, and anorexia nervosa.

Measurement of sex hormones

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When determining levels of circulating estradiol or testosterone, either a total measurement could be done that includes the "free" and the bound fractions, or only the "free" hormone could be measured.

See also

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ABP

External links

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Glycoproteins