Everything about Sex Hormone-binding Globulin totally explained
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| Name = Sex hormone-binding globulin
| HGNCid = 10839
| Symbol = SHBG
| AltSymbols =; ABP; MGC126834; MGC138391
| OMIM = 182205
| ECnumber =
| Homologene = 813
| MGIid = 98295
| GeneAtlas_image1 = PBB_GE_SHBG_215689_s_at_tn.png
| Function =
| Component =
| Process =
| Orthologs =
}}
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
These sex hormones circulate in the bloodstream, bound mostly to SHBG and to some degree bound to
serum albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a
cell and activate its
receptor. The SHBG inhibits the function of these hormones. Thus bioavailability of sex hormones is influenced by the level of SHBG.
SHBG production
SHBG is produced by the
liver cells and is released into the bloodstream. Other sites that produce SHBG are the brain, uterus, and placenta and vagina. In addition SHBG is produced by the testes; testes-produced SHBG is also called
androgen-binding protein. 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 is decreased by high levels of
insulin and insulin-like growth factor 1 (
IGF-1). Also, high
androgen levels decrease SHBG, while high
estrogen and
thyroxine levels increase it.
However, recent evidence suggests that it's the livers production of fats that reduces SHBG levels, not any direct effect of insulin and specific genetic mechanisms have been found that do this.
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. There has recently been research to link high SHBG levels with breast and testicular cancer as well.
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.
A
free androgen index expresses the ratio of testosterone to the sex hormone binding globulin and can be used to summarise the activity of free testosterone.
Further Information
Get more info on 'Sex Hormone-binding Globulin'.
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