sex hormone-binding globulin (SHBG)

Sex hormone-binding globulin, or SHBG, is the carrier protein that grabs and ferries sex steroids — testosterone and estradiol — through the blood. By holding most of these hormones bound and inactive, it acts as a gatekeeper that controls how much is actually free to reach the tissues.

SHBG is made in the liver, and its level is far from fixed. Estrogen, thyroid hormone, and pregnancy push it up; high insulin, obesity, and androgens push it down. Because it binds testosterone more tightly than estradiol, a change in SHBG does not affect both hormones equally — a fall in SHBG frees up relatively more testosterone, tilting the balance.

This is why SHBG matters clinically. In conditions like polycystic ovary syndrome, high insulin lowers SHBG, raising free androgens and worsening symptoms even when total testosterone looks normal. Doctors sometimes use SHBG together with total testosterone to estimate the free, biologically meaningful amount.