What is sex hormone binding globulin (SHBG)?
Sex hormone binding globulin (SHBG) is a protein produced by the liver that binds to testosterone and other sex hormones in the bloodstream, reducing the amount of free testosterone available to body tissues.
SHBG is a protein made by your liver that attaches to testosterone molecules circulating in your blood. When testosterone binds to SHBG, it becomes unavailable to body tissues and cells, meaning only the unbound testosterone, called "free testosterone," can produce biological effects. This distinction matters because total testosterone alone does not tell the full picture of hormonal status.
The amount of SHBG in your bloodstream directly affects how much free testosterone you have available. High SHBG levels can reduce free testosterone even when total testosterone appears adequate, because more of the hormone gets locked onto the protein. Low SHBG levels increase free testosterone availability, potentially making more hormone active in your body. For this reason, testosterone replacement therapy providers track both total and free testosterone levels when adjusting dosing and monitoring treatment response.
SHBG levels can change based on age, weight, liver function, insulin sensitivity, and certain medications or conditions. Higher SHBG is sometimes associated with aging, liver disease, or thyroid problems, while lower SHBG can relate to obesity, insulin resistance, or polycystic ovary syndrome. Understanding your SHBG level helps TRT providers interpret blood work accurately and personalize your treatment plan.