What is SHBG (sex hormone binding globulin)?
SHBG is a protein produced by the liver that binds to testosterone and estrogen in the bloodstream, determining how much of these hormones remain free and biologically active.
Sex hormone binding globulin (SHBG) is a carrier protein synthesized in the liver that attaches to testosterone and estrogen molecules in the blood. Once bound to SHBG, these hormones are essentially inactive and unavailable for the body's tissues to use. Only the unbound, or "free," portion of these hormones can produce biological effects.
Total testosterone or estrogen levels alone can be misleading without knowing SHBG status. A patient might have adequate total hormone on lab work, but if SHBG is elevated, most of it is bound and unavailable, resulting in low free hormone levels and associated symptoms like fatigue, low libido, or mood changes. Conversely, when SHBG is low, a higher percentage of total hormone circulates freely, making the same total level more biologically active.
TRT clinics order SHBG tests to understand the complete hormonal picture during evaluation and treatment monitoring. High SHBG can indicate liver disease, hyperthyroidism, or certain medications and may require dosage adjustments. Low SHBG occurs with obesity, insulin resistance, and hypothyroidism. By measuring SHBG alongside total and free testosterone levels, practitioners at hormone therapy clinics gain insight into whether symptoms reflect true hormone deficiency or simply poor hormone availability, helping guide appropriate treatment decisions.