Free Testosterone and bio available testosterone can be found in the blood. It binds itself to a protein called Sex Steroid Binding Globulin (SSBG) or you can call it Sex Hormone binding Globulin (SHBG). Free available testosterone refers to the ammount of free testosterone plus albumin bound testosterone. Basically it is the ammount of testosterone not bound to SHBG.

Usually it is said that bio available testosterone is a fraction of circulating testosterone that is ready to go into cells and examines the bioactivity of testosterone itself , rather just a measurement of total serum testosterone. A decreased ammount of sex Hormone binding globulin can be evident when dealing with obesity , over the counter androgen use , hypothyroidism , increased estrogen levels. Free testosterone is also hard to measure accurately.

Concentration of free testosterone is usually pretty low , typically less than 2% of the total ammount of testosterone. To find out how much bio available testosterone is in your system , one can get a blood test where they will extract a small blood sample.