Testosterone is an anabolic steroid hormone synthesized from androstenedione in the Leydig cells of the testes of males and, in smaller quantities, in the ovaries of females. (1) Small amounts are also secreted by the adrenal glands in both sexes.  In both men and women a large portion of total testosterone production occurs in peripheral tissues by conversion of circulating DHEA-S, DHEA, and androstenedione.   In post-menopausal women, the ovaries and peripheral tissues continue to produce testosterone and other androgens, which then serve as precursors for the synthesis of estradiol in the peripheral tissues. The conversion of precursors into testosterone and the estrogens in peripheral tissues allows these steroids to be delivered to the appropriate tissues without leakage of significant amounts into the circulation, avoiding undesirable effects of high circulating levels. (2,3,4)  Testosterone exhibits a diurnal rhythm, with highest levels in the morning and a nadir around  midnight. (5,6) In men, testosterone plays an important role in the development of male reproductive tissues including the testis and prostate, as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and hair growth. (7,8)  Additionally, testosterone is essential for health and well-being, stamina, sexual function, cardiovascular health, and immune protection. (9,10,11,12)  Testosterone measurements are typically used for clinical evaluation of hypogonadism in males and hyperandrogenic states in females. (13,14,15) In blood, only 1 to 15% of testosterone is in its unbound or biologically active form.  The remaining testosterone is bound to serum proteins. Unbound testosterone enters the saliva via intracellular mechanisms, and in saliva the majority of testosterone is not protein-bound. (16)  The serum-saliva correlation for testosterone is very high for males, but only modest for females, possibly because women’s values often fall near the bottom of the measurable range for both serum and saliva immunoassay kits. (17,18)