How is testosterone measured?
If a doctor suspects someone is not producing enough testosterone, he/she will check if the total blood testosterone level falls into the acceptable range. The doctor also may instruct the laboratory to measure the amount of free or loosely bound testosterone (about 40 percent of the total testosterone is strongly bound to a protein called sex hormone binding globulin, known as SHBG; about 58 percent is weakly bound to another protein called albumin) and the amount of free testosterone (only about two percent circulates freely in the blood). Blood levels of SHBG increase with age, so older men may have a higher percentage of bound testosterone and a lower percentage of free testosterone. Bioavailable testosterone includes the non-SHBG bound testosterone or the sum of the testosterone, which is bound to albumin and free (unbound) testosterone.
