There are several blood tests used to measure or monitor testosterone levels in men. The most common is total testosterone. However, more than 50% of total testosterone in the circulation is tightly bound to a carrier molecule called SHBG (sex hormone binding globulin), which renders this portion unavailable to the cells that need it. Thus, many experts rely on a test called free testosterone, which is a better indicator of the levels of testosterone that are bioavailable. Free testosterone can be measured directly as a radioimmunoassay (offered by most labs), or calculated using values for total testosterone and SHBG. A test called equilibrium dialysis is mentioned frequently in the literature as the “gold standard” technique for measuring free testosterone, but it is labor-intensive and its use is primarily restricted to research laboratories.
Additional blood tests that may be ordered by the physician to assess the hormonal profile in men include LH, FSH, estradiol, and prolactin.