Free Testosterone (FT) is the active form of testosterone (T) that can enter the cell. The main binding of Testosterone is to SHBG and albumin. There are different ways to determine FT. Some use free androgen index method, which is done by a calculation formula. Recently the LC/MS-MS (liquid chromatography / mass spectroscopy) has become the most widely and easy-to-use method. Free T is usually 2 percent or greater of total T. Values under 2 percent are associated with higher SHBG or albumin.
As shown in the figure above, about 2 percent of the testosterone in the body is active. This “free testosterone” is not attached to binding proteins that would prevent it from interacting with its receptor.
About 40 percent of the body's testosterone is attached to albumin. This is a protein that can release the hormone as the need for it arises in the body. Free testosterone and testosterone attached to albumin are referred to as “bioavailable testosterone.”
In a healthy young male, about 60 percent of his testosterone is attached to sex hormone-binding globulin (SHBG). Hormones bound to SHBG can't be used by the body and lose their anabolic effect. As males grow older or if an illness is present, SHBG sweeps up more and more testosterone, lowering free testosterone and its benefits.
Total testosterone is the sum of bioavailable testosterone and testosterone bound to SHBG. Measuring the total testosterone in the blood may not provide the whole picture and let you know how much “active” or usable testosterone you have.
A new study indicates that measuring free testosterone and total testosterone may provide a more accurate picture than measuring total testosterone levels alone when diagnosing a deficiency of this male hormone. There is concern that, under current clinical guidelines, some men are misdiagnosed with testosterone deficiency and receive inappropriate therapy. In contrast, others who should get treatment don't receive it, D.r Leen Antonio (University of Leuven, Belgium) explained when presenting her research here at the European Congress of Endocrinology (ECE) 2015.
"I think it would be preferable to use free testosterone rather than total testosterone to measure deficiency, especially in men with borderline total testosterone levels. Our results suggest that free testosterone is more informative than total testosterone," she told attendees.
Currently, total-testosterone levels are used to measure testosterone deficiency or hypogonadism. However, she explained that only free testosterone, unbound by sex-hormone–binding globulin (SHBG), can enter and activate androgen receptors in cells, resulting in testosterone-mediated effects.