SHBG
SHBG, sex hormone binding globulin, is a protein in the blood that grabs hold of testosterone and estradiol. Hormone that is bound to SHBG is not available to the body's cells, so the amount of SHBG changes how much of your total hormone level is actually doing anything.
Where you'll meet it
On more detailed blood results, and in conversations where the total testosterone level and how someone feels do not seem to match.
Total and free
A standard blood test measures total testosterone or estradiol, bound and unbound together. The unbound part, called free, is the part cells can use. If SHBG is high, more is bound and less is free; if it is low, the reverse. Two people with the same total testosterone can have quite different free levels. Some labs calculate a free testosterone figure from the total, SHBG and albumin.
What changes it
Estrogen, particularly taken by mouth, raises SHBG, because it passes through the liver where SHBG is made. Testosterone tends to lower it. Thyroid problems, liver health and body weight also shift it. It is one reason oral and transdermal estrogen can give different results at similar doses. Most people never need to think about it; it earns its place when the numbers stop making sense.
Related pages
Not sure where to begin?
The Start Here guide covers what hormones change, how people get a prescription, what the blood tests are for and what it costs.
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