Sex hormone binding globulin
From Biocrawler, the free encyclopedia.
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.
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Transport of sex hormones
These sex hormones circulate in the blood stream mostly bound to SHBG, to some degree bound to albumen, and 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 the level of SHBG.
SHBG production
SHBG is produced by the liver cells and released into the blood stream. The gene for SHBG is located on chromosome 17.
Control
SHBG levels appear to be controlled by a delicate balance of enhancing and inhibiting factors. Its level are decreased by high levels of insulin and insulin-like growth factor-I (IGF-I), as well as high body weight. In women with low insulin levels and low body weight SHBG levels are elevated. Also, androgen levels decrease, and estrogen and thyroxine levels increase SHBG.
Conditions with high or low levels
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
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.
