Testosterone is the main hormone responsible for the development of secondary sex characteristics in men. Testosterone is produced mainly in the testicles and partially by the adrenal glands. In women, low levels of testosterone are produced by the adrenal glands. In men, about half of blood testosterone is present in an inactive form (bound to SHBG), and the other half is present in a free form or weakly bound to albumin. The total free and albumin-bound forms constitute bioavailable testosterone. Total testosterone levels are generally an accurate representation of the biological activity of testosterone in the tissues. Certain conditions may interfere with this interpretation. In such cases, free or bioavailable testosterone levels are clinically more useful. The results of this bioavailable testosterone test were obtained by ammonium sulfate precipitation.
In men, low levels of bioavailable testosterone are consistent with primary hypogonadism (testicular impairment) or secondary to a pituitary problem. Andropause in men (decreased libido, erectile dysfunction, muscle weakness, etc.) is caused by secondary hypogonadism associated with aging. In women, high levels of bioavailable testosterone are consistent with overactive adrenal glands or ovaries (polycystic ovaries).