SHBG is the blood transport protein for testosterone and oestradiol. It is a large glycoprotein with a molecular weight of about 95 kD, and exists as a homodimer composed of two identical subunits. SHBG has a high affinity to dihydrotestosterone, medium affinity to testosterone and oestradiol, and only a low affinity to estrone, DHEA, androstendione, and oestradiol. Albumin, which exists in higher concentrations than SHBG, also binds sexual steroids although, has a lower affinity.
SHBG has a half life of around 7 days and is produced mainly by the liver. Its synthesis and secretion are regulated by oestrogen.
Decreased SHBG are associated with conditions where elevated androgen levels are present or where the effect of androgen on its target organ is excessive. This explains the gender-related differences seen between men and women, especially during puberty.
SHBG is a useful supplementary parameter in the determination of androgen where a relatively high concentration of free androgen (e.g testosterone) is suspected. By calculating the free androgen index (FAI), from the ratio of total testosterone (TT) to SHBG (TT/SHBG*100), it is possible to calculate the approximate amount of free testosterone.
Elevated SHBG levels can be seen in elderly men, and are often found in patients with hyperthyroidism and cirrhosis of the liver. SHBG levels also increase when oral contraceptives or antiepileptic drugs are taken. Pregnant women have markedly higher SHBG serum concentrations due to their increased oestrogen production. Decreased SHBG concentrations are often seen with hypothyroidism, polycystic ovarian syndrome (PCOS), obesity, hirsuitism, elevated androgen levels, alpocia and Acromegaly.
Male: 13 – 71 mmol/l
Female: 18 – 114 mmo/L
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