Also known as: Free Testosterone (Direct) and Total, Serum.

Testosterone is a steroid hormone (androgen) that is produced by special endocrine tissue (the Leydig cells) in the male testes. Its production is controlled and controlled by luteinizing hormone (LH), which is manufactured in the pituitary gland. Testosterone works within a negative feedback mechanism, so as testosterone increases, LH decreases, while increased LH causes decreased testosterone. Testosterone levels are diurnal and peak in the early morning hours (about 4:00 to 8:00 am), and have the lowest levels in the evening (about 4:00 to 8:00 pm). Levels increase after exercise as well, but decrease with age. Nearly two-thirds of testosterone circulates in the blood bound to sex-hormone binding protein and slightly less than one-third is bound to albumin. A small percent circulates in the blood as free testosterone. The concentration of free testosterone is very low, normally <2% of the total testosterone concentration. In most women and men, >50% of total circulating testosterone is bound to sex hormone-binding globulin, SHBG, and most of the remaining is bound to albumin. Routinely available assay methods that are used to measure total testosterone are not sensitive enough to accurately quantitate the free testosterone fraction directly. Free testosterone is estimated in this particular test by a direct, analogue radioimmunoassay method. This assay uses a labeled testosterone analogue that has a low binding affinity for both albumin and SHBG but is bound by antitestosterone antibody used in the assay. Since the analogue is unbound in the plasma, it then competes with free testosterone for binding sites on an antitestosterone antibody that is immobilized on the surface of the polypropylene tube.