This test is used to help interpret abnormal testosterone levels in men and women.
SHBG is a protein produced in the liver. Its role is to bind firmly with the sex hormones (testosterone and estradiol) in order to limit their action. For a given total testosterone level, the higher the SHBG levels, the less free testosterone there will be (active at the cellular level). Conversely, the lower the SHBG levels, the more free and active testosterone there will be.
SHBG levels are therefore very useful when the total testosterone levels measured in the blood do not correlate with the clinical situation. A measurement of SHBG level in the blood is therefore very often ordered at the same time as the testosterone level, and helps calculate the amount of free testosterone and bioavailable testosterone in men and women (see those headings).
A number of conditions can affect SHBG levels, including liver disease, thyroid gland disorders, pregnancy, obesity, and so on.