The test measures the amount of Sex Hormone Binding Globulin in blood. SHBG is a protein (globulin) produced by the liver that is able to strongly bind testosterone and estradiol. SHBG then transports these hormones in an inactive form through the body via the blood. Testosterone inhibits the production of SHBG whereas estradiol actually stimulates production.
Men produce mostly testosterone in the testes and very little estradiol. Women produce mostly estradiol in the ovaries and a little testosterone in ovaries and in the adrenal gland. Change in the amount of SHBG affects the amount of testosterone that is freely available.
At an increased amount of SHBG there is probably less testosterone freely available than expected based on the total testosterone test.
At a reduced amount SHBG is more likely to have more testosterone freely available than expected based on the total testosterone test.