SHBG is a protein produced by the liver that binds testosterone with high affinity, making it unavailable for tissue use. Higher SHBG means more bound testosterone and less free testosterone. SHBG is influenced by age, body composition, thyroid function, insulin resistance, and medications.
Low SHBG (common in obese or insulin-resistant men) can make total T appear normal while free T is actually adequate. High SHBG (common in lean, older men) can make total T appear adequate while free T is deficient.