As the principal glucocorticoid in humans, cortisol plays a crucial role in modulating the metabolic and homeostatic process that protects against stress, shock, inflammation, etc.3-7) It is synthesized primarily in the zona fasciculata with a small contribution from the zona reticularis.3) While cortisol is essentially secreted by the adrenal glands, cortisone is mainly produced using 11b-hydroxysteroid dehydrogenase isoenzymes, which interconvert cortisol to hormonally inactive cortisone.3) Cortisol and cortisone are extensively metabolized to tetrahydro-reduced derivatives: tetrahydrocortisol (THF) and tetrahydrocortisone (THE), and their 5a-stereoisomer (allo-THF and allo-THE, respectively). 3,[7][8][9] Metabolism decreases the biological activity of hormones and increases their water solubility by converting them to hydrophilic compounds that can be excreted in urine. Unmetabolized cortisol and cortisone comprise only ca. 0.1% of the total urinary cortisol metabolites. At least 90% of the tetrahydro-derivatives of cortisol and cortisone metabolites are excreted into the urine as sulfate or glucuronide conjugates.10-13) Tetrahydro-11-deoxycortisol (THS) and its 5a-stereoisomer (allo-THS) which are tetrahydro-reduced metabolites of 11-deoxycortisol (11-DOC), a biosynthetic precursor of cortisol, are also excreted in urine as conjugated forms. 8,10,12) In order to obtain information regarding the related biochemistry, physiology, and pathophysiology of endocrine disorders, it is necessary to identify all the corticosteroid metabolites found in urine, and determine the exact structure of their conjugates, their concentration, and the dynamics of their formation and disposal.At present, analytical methods for the detection of conjugated tetrahydrocorticosteroids are based on gas chromatography and mass spectrometric determination of hydrolyzed and derivatized compounds.14-18) Although these methods are robust and sensitive, sample preparation in these indirect methods is time consuming, and the GC-MS sample throughput is relatively low. Thus the development or more straightforward methods based on the direct analysis of steroid conjugates without the need for a deconjugation process is of great interest. The application of liquid chromatographic separation interfaced by soft ionization techniques, such as electrospray ionization (ESI) with tandem mass spectrometry and/or ion trap mass spectrometry, offers an effective analytical tool for the direct monitoring of the urinary conjugates of tetrahydrocorticosteroids. 19,20) Recently, we reported a highly sensitive and specific liquid chromatography/ESI-linear ion trap mass spectrometry method for the direct measurement of the glucuronide conjugates of THF, THE, THS, and their 5a-stereoisomer in human urine.21) The authentic glucuronides had been chemically synthesized by Hosoda et al. [22][23][24][25] One significant drawback of sulfate conjugate analysis, however, is still the lack of reference materials, which are essential for the development and application of ...