Sex Hormone Binding Globulin (SHBG)
Solid-phase, two-site chemiluminescent immunometric assay
1-3 business days
Sex hormone binding globulin (SHBG) is a ~100 kDa homodimeric plasma protein where its 373 amino acid monomeric form is ~50 kDa. The gene for SHBG is located on chromosome 17p1.
SHBG serves as the plasma transport protein for testosterone and dihydrotestosterone (DHT). Estrogen is also carried on SHBG whereas the affinity of SHBG for estrogen is lower than for testosterone or dihydrotestosterone. Estrogens and thyroid hormone increase SHBG concentrations, whereas insulin, growth hormone, glucocorticoids, androgens, and progestins lower SHBG concentrations. SHBG concentrations are higher in children than in adults. SHBG concentrations are generally higher in women than in men although their reference intervals overlap substantially. SHBG levels may decline in persons with the metabolic syndrome. With increasing age SHBG levels rise in men.
Testosterone exists in 3 pools: free, loosely (a.k.a. - weakly) bound to albumin and tightly bound to SHBG. Both free and weakly bound testosterone are considered to be biologically active (e.g., "bioactive" or "bioavailable"). Various sources report that the percent free testosterone (as a proportion of the total testosterone) is 0.5 - 3.0% or 1.0 - 4.0%. In men, albumin-bound testosterone is 50 - 68% of the total testosterone and SHBG-bound testosterone is 30 - 45% of the total testosterone. In women, albumin-bound testosterone is ~25% of the total testosterone and SHBG-bound testosterone is ~70% of the total testosterone.
Free or bioavailable testosterone can be measured using a variety of methods: calculated from the total testosterone and SHBG measurements, ultracentrifugation, dialysis equilibrium, precipitation of SHBG-bound testosterone, direct immunoassay and mass spectroscopy. What constitutes the "best method" for measuring free or bioavailable testosterone is controversial; however, many experts believe that in the setting of the routine clinical laboratory, the calculation of the free testosterone based upon the total testosterone and SHBG levels is the superior approach (see: http://www.issam.ch/freetesuit.htm) . Note that in women and children, total testosterone is best measured by mass spectroscopy.
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Sample Volume: 1 mL
Minimum Volume: 0.2 mL (Repeat testing is not possible with this specimen volume.)
Storage: Centrifuge, separate and transfer the serum to a plastic vial. Ship specimens with a frozen cold pack for overnight delivery.
Stability (collection to time of analysis/testing):
- Refrigerated: 7 days
- Frozen: 2 months
- Non-pregnant females: 18 - 144 nmol/L
- Males: 10 - 57 nmol/L
EPIC Test Name: SEX HORMONE BINDING GLOBULIN