What is SHBG (sex hormone-binding globulin)?
SHBG is a protein produced primarily by the liver that binds to testosterone and other sex hormones, affecting how much testosterone remains free and active in the body.
Sex hormone-binding globulin is a transport protein made mainly by the liver that attaches to testosterone molecules circulating in your blood. When testosterone binds to SHBG, it becomes unavailable to cells and tissues. Only unbound testosterone (called free testosterone) can enter cells and produce biological effects.
This distinction matters for therapy interpretation. A man might have a total testosterone level in normal range, but if SHBG is elevated, much of that testosterone is bound and inactive. Conversely, low SHBG means a higher proportion of testosterone remains free and available. SHBG levels vary based on age, body composition, liver function, estrogen levels, and thyroid status.
When reviewing lab work, many testosterone replacement therapy providers order SHBG alongside total and free testosterone tests. This three-part picture clarifies whether low symptoms stem from truly low testosterone production or from high SHBG sequestering available hormone. Some conditions and medications raise SHBG (liver disease, hyperthyroidism, estrogen exposure), while others lower it (obesity, insulin resistance, hypothyroidism). Interpreting SHBG helps clinicians determine appropriate dosing and identify underlying metabolic issues that might need separate treatment.