What is the difference between free testosterone and bioavailable testosterone?
Testosterone is present in the blood as “free” testosterone (2-3%) or bound testosterone. The latter may be bound to either albumin (a serum protein) or to a specific binding protein called Sex Steroid Binding Globulin (SSBG) or Sex Hormone Binding Globulin (SHBG). The binding of testosterone to albumin is not very tight and is easily reversed; so the term bioavailable testosterone (BAT) refers to the sum of free testosterone plus albumin-bound testosterone. Alternatively, it is the fraction of circulating testosterone that is not bound to SSBG. It is suggested that BAT represents the fraction of circulating testosterone that readily enters cells and better reflects the bioactivity of testosterone than does the simple measurement of serum total testosterone. Also, varying levels of SSBG can result in inaccurate measurements of BAT. Decreased SSBG levels can be seen in obesity, hypothyroidism, androgen use, and nephritic syndrome. Increased levels are seen in cirrhosis, hyperthyroidism,
