Total, free or bioavailable testosterone: What does it all mean?

Testosterone circulates in the blood in three forms:

1. Nearly half is bound tightly to a protein called SHBG (sex hormone binding globulin). In this form, testosterone is unable to enter cells.
2. The other half is more weakly bound to albumin. In this form, testosterone can eventually detach from albumin and increase the level of biologically active testosterone.
3. A very small fraction called “free testosterone” is not bound to any protein and can therefore freely enter cells and be used by the body.

The total quantity of albumin-bound and free forms is called “bioavailable testosterone.” Total testosterone is the sum of the three forms of testosterone.

In the vast majority of cases, measuring the level of any fraction will provide the required clinical information. However, there are cases in which the total testosterone level is less representative of biological activity. When this happens, free or bioavailable testosterone must be measured.

Regardless of the fraction measured, it should be remembered that testosterone levels vary throughout the day. In young men, these levels can as much as double between a morning and mid-afternoon sample. Show up for your appointment on time!