The carrier that sets free hormone levels
Sex hormone binding globulin (SHBG) is a protein made mainly by the liver that binds sex steroids in the blood, with a particularly tight grip on testosterone and a weaker hold on estradiol. Hormone bound to SHBG is largely held in reserve, while the small unbound (free) fraction is what is generally considered available to tissues. So SHBG acts as a buffer that influences how much active hormone reaches cells.
Because of this, total testosterone can look normal while free testosterone is altered if SHBG is unusually high or low, which is why clinicians sometimes measure or calculate the free fraction.
What raises and lowers SHBG
SHBG tends to rise with higher estrogen states, thyroid hormone excess, aging in men, and liver-related factors, and to fall with higher insulin levels, obesity, and excess androgens. This means SHBG shifts can change free hormone availability without any change in gland output.
Interpreting SHBG always requires the full clinical picture rather than the number alone. This is general educational information, not medical advice; hormone testing should be interpreted by a qualified professional.