Title of Invention: Method for differentially diagnosing ACTH-dependent Cushing's syndrome Abstract: This invention provides for an improved method for differentially diagnosing ACTH-dependent Cushing's syndrome. Current practice for differentially diagnosing ectopic ACTH syndrome and Cushing's Disease measures relative ACTH concentrations from the inferior petrosal venous sinus compared to fluid obtained from a periphery venous sample. This is performed before and after administration of exogenous corticotropin releasing factor, or after administration of metyrapone. This invention uses glucocorticoid receptor antagonists to induce release of endogenous CRH which stimulates ACTH to increase in patients with ectopic ACTH syndrome but not in those with Cushing's Disease. Detailed Description: Part general_description_paragraphs: Part CROSS-REFERENCES TO RELATED APPLICATIONS: This application is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. patent application Ser. No. 15/987,365, filed May 23, 2018, which is a continuation of Ser. No. 15/796,443, filed Oct. 27, 2017 (now U.S. Pat. No. 10,006,924, issued Jun. 26, 2018), which is a continuation of U.S. patent application Ser. No. 15/236,015, filed Aug. 12, 2016, (now U.S. Pat. No. 9,829,495, issued Nov. 28, 2017), which application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 62/204,723, filed Aug. 13, 2015, the entire contents of which applications are hereby incorporated by reference in their entireties. Part BACKGROUND OF THE INVENTION: Cortisol is a steroid produced by the adrenal glands and is used in the body to respond to physical and emotional stress and to maintain adequate energy supply and blood sugar levels. Cortisol production is highly regulated by the hypothalamic-pituitary-adrenal axis (HPA) through a complex set of direct influences and negative feedback interactions. In healthy individuals, insufficient cortisol in the bloodstream triggers the hypothalamus to release corticotropin-releasing hormone (CRH) which signals to the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn stimulates the adrenal glands to produce more cortisol. Excessive cortisol inhibits hypothalamus from producing CRH, thus inhibiting the pituitary gland from releasing ACTH, which in turn suppresses cortisol production. The HPA regulation also results in a diurnal rhythm of cortisol levels, reaching peaks in the morning and nadirs around midnight. Pathological conditions associated with the HPA can affect the diurnal rhythm of the cortisol and ACTH production and cause serious health problems. Cushing's syndrome is one of these problems. Patients having Cushing's syndrome usually have easy bruising; abdominal obesity and thin arms and legs; facial plethora; acne; proximal muscle weakness; and/or red purple stripes across the body. Cushing's syndrome is accompanied by hypercortisolemia, a condition involving a prolonged excess of circulating cortisol. Cushing's syndrome can be classified as exogenous Cushing's syndrome, which is caused by excess use of glucocorticoids drugs, such as prednisone, dexamethasone, and hydrocortisone, and endogenous Cushing's syndrome, which is caused by deregulatory abnormalities in the HPA axis. Endogenous Cushing's syndrome consists of the ACTH-independent Cushing's syndrome, characterized by an overproduction of cortisol in the absence of elevation of ACTH secretion; the ACTH-dependent Cushing's syndrome, characterized by excessive ACTH secretion. ACTH-dependent Cushing's syndrome includes roughly 80% of patients having endogenous Cushing's syndrome and consists of two major forms: Cushing Disease and ectopic ACTH syndrome. The former is caused by a pituitary tumor and the latter is caused by a tumor outside the pituitary. Correct differential diagnosis between the Cushing Disease and ectopic ACTH syndrome is important for endocrinologists to recommend transphenoidal surgery or appropriate imaging to identify source of the ectopic ACTH secretion. One current approach of differentially diagnosing patients with ACTH-dependent Cushing's syndrome involves measuring ACTH levels from samples obtained simultaneously from both inferior petrosal venous sinus (IPS)—a procedure referred to as inferior petrosal venous sinus sampling (IPSS)—and from the internal jugular or another peripheral vein. In one approach, referred herein as CRH-IPSS, 5 blood samples are taken from each IPS and the internal jugular vein, two before and three after administration of CRH. A central-to-periphery ACTH ratio of >2 before and >3 after the administration of CRH is consistent with Cushing Disease while a lower ratio favors ectopic ACTH syndrome. This procedure requires prolonged catheterization with the likelihood of infection, thrombosis, or bleeding rising with the duration of catheterization. In addition CRH is a protein which is expensive to produce, causing a shortage in supply between 2011 and early 2013, and requires sophisticated handling. Thus, the results from CRH-IPSS for differentially diagnosing patients with ACTH-dependent Cushing's syndrome often fall in the gray area. Desmopressin acetate (DDAVP), the alternative to CRH, which has also been used for IPSS, has similar disadvantages. Another approach, referred to herein as metyrapone-IPSS, is similar to the one above, except that metyrapone instead of CRH is administered to the patient before IPSS and that samples are only taken from the patients after the metyrapone administration. Although metyrapone-IPSS improves the CRH-IPSS—since it dispenses with the need for sampling before the administration of metyrapone, and thus reduces the duration of catheterization and likelihood of infection, thrombis, or bleeding associated therewith—it also has serious limitations. First, metyrapone acts to block the conversion of 11-deoxycortisol to cortisol by 11β-hydroxylase, causing a decrease in cortisol level, which in turn stimulates ACTH production and release. Since its effect on the ACTH secretion is indirect, the test result may be skewed by other factors affecting the cortisol synthesis. Second, as a cortisol synthesis blocker, treatment of metyrapone—especially at a high dose—may result in adrenal insufficiency or have deleterious effects on various normal bodily functions that require cortisol—for example, the anti-stress and anti-inflammation functions. Third, metyrapone is currently not available in the United States, consequently this diagnosis method is out of reach for many patients in this country. Part BRIEF SUMMARY OF THE INVENTION: In a first aspect, provided herein is a method of differentially diagnosing adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome in a patient with hypercortisolemia where the differential diagnosis is between ectopic ACTH syndrome and Cushing Disease. The method comprises: (i) selecting a patient with Cushing's syndrome and elevated ACTH levels; (ii) administering a dose of glucocorticoid receptor antagonist (GRA) sufficient to increase ACTH from the pituitary gland by at least two fold in persons with normal HPA function; (iii) waiting for at least two hours; and (iv) obtaining from the patient an ACTH concentration ratio, which is derived both from the ACTH concentrations in fluid obtained from either the left or right inferior petrosal venous sinus and from fluid obtained from a periphery vein, e.g., a jugular vein. The patient is diagnosed with Cushing Disease if the ACTH concentration ratio is greater than 3. In some embodiments, the periphery venous sample is a jugular venous sample. In some embodiments, the ratio is derived from the ACTH concentration in fluid obtained from the left and right inferior petrosal venous sinuses. In some embodiments, the GRA is a selective inhibitor of the glucocorticoid receptor. In some cases, the first and second samplings of ACTH are taken 5-10 minutes apart from both the inferior petrosal venous sinus and a periphery venous sample. In some cases, the GRA is a selective inhibitor of the glucocorticoid receptor. In some embodiments, the GRA comprises a steroidal backbone with at least one phenyl-containing moiety in the 11-β position of the steroidal backbone. In some cases, the phenyl-containing moiety in the 11-β position of the steroidal backbone is a dimethylaminophenyl moiety. In some cases, the GRA is mifepristone. In some embodiments, the GRA is selected from the group consisting of 11β-(4-dimethylaminoethoxyphenyl)-17α-propynyl-17β-hydroxy-4,9 estradien-3-one and (17α)-17-hydroxy-19-(4-methylphenyl)androsta-4,9(11)-dien-3-one. In some embodiments, the glucocorticoid receptor antagonist is (11β,17β)-11-(1,3-benzodioxol-5-yl)-17-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one. In some embodiments, the GRA has a non-steroidal backbone. In some cases, the GRA backbone is a cyclohexyl pyrimidine. In some cases, wherein the cyclohexyl pyrimidine has the following formula: the dashed line is absent or a bond; X is selected from the group consisting of O and S; R1 is selected from the group consisting of cycloalkyl, heterocycloalkyl, aryl, and heteroaryl, optionally substituted with 1-3 R1a groups; each R1a is independently selected from the group consisting of H, C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, C1-6 alkoxy, C1-6 alkyl OR1b, halogen, C1-6 haloalkyl, C1-6 haloaloxy, OR1b, NR1bR1c, C(O)R1b, C(O)OR1b, OC(O)R1b, C(O)NR1bR1c, NR1bC(O)R1c, SO2R1b, SO2NR1bR1c, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl; R1b and R1c are each independently selected from the group consisting of H and C1-6 alkyl; R2 is selected from the group consisting of H, C1-6 alkyl, C1-6 alkyl-OR1b, C1-6 alkyl NR1bR1c and C1-6 alkylene heterocycloalkyl; R3 is selected from the group consisting of H and C1-6 alkyl; Ar is aryl, optionally substituted with 1-4 R4 groups; each R4 is independently selected from the group consisting of H, C1-6 alkyl, C1-6 alkoxy, halogen, C1-6 haloalkyl, and C1-6 haloalkoxy; L1 is a bond or C1-6 alkylene; and subscript n is an integer from 0 to 3, or salts and isomers thereof. In some cases, the GRA backbone is a fused azadecalin. In some cases, the fused azadecalin is a compound having the following formula: wherein L1 and L2 are members independently selected from a bond and unsubstituted alkylene; R1 is a member selected from unsubstituted alkyl, unsubstituted heteroalkyl, unsubstituted heterocycloalkyl, —OR1A, NR1CR1D, —C(O)NR1cR1D, and —C(O)OR1A, wherein R1A is a member selected from hydrogen, unsubstituted alkyl, and unsubstituted heteroalkyl; R1c and R1D are members independently selected from unsubstituted alkyl and unsubstituted heteroalkyl, and are optionally joined to form an unsubstituted ring with the nitrogen to which they are attached, wherein said ring optionally comprises an additional ring nitrogen. R2 has the formula: wherein R2G is a member selected from hydrogen, halogen, unsubstituted alkyl, unsubstituted heteroalkyl, unsubstituted cycloalkyl, unsubstituted heterocycloalkyl, —CN, and —CF3; J is phenyl; t is an integer from 0 to 5; X is —S(O2)—; and R5 is phenyl optionally substituted with 1-5 R5A groups, wherein R5A is a member selected from hydrogen, halogen, —OR5A1, S(O2)NR5A2R5A3, —CN, and unsubstituted alkyl, and R5A1 is a member selected from hydrogen and unsubstituted alkyl, and R5A2 and R5A3 are members independently selected from hydrogen and unsubstituted alkyl, or salts and isomers thereof. In some cases, the GRA backbone is a heteroaryl ketone fused azadecalin or an octahydro fused azadecalin. In some cases, the heteroaryl ketone fused azadecalin has the formula: wherein R1 is a heteroaryl ring having from 5 to 6 ring members and from 1 to 4 heteroatoms each independently selected from the group consisting of N, O and S, optionally substituted with 1-4 groups each independently selected from R1a; each R1a is independently selected from the group consisting of hydrogen, C1-6 alkyl, halogen, C1-6 haloalkyl, C1-6 alkoxy, C1-6 haloalkoxy, CN, N-oxide, C3-8 cycloalkyl, and C3-8 heterocycloalkyl; ring J is selected from the group consisting of a cycloalkyl ring, a heterocycloalkyl ring, an aryl ring, and a heteroaryl ring, wherein the heterocycloalkyl and heteroaryl rings have from 5 to 6 ring members and from 1 to 4 heteroatoms each independently selected from the group consisting of N, O, and S; each R2 is independently selected from the group consisting of hydrogen, C1-6 alkyl, halogen, C1 6 haloalkyl, C1 6 alkoxy, C1-6 haloalkoxy, C1-6 alkyl-C1-6 alkoxy, CN, OH, NR2aR2b, C(O)R2a, C(O)OR2a, C(O)NR2aR2b, SR2a, S(O)R2a, S(O)2R2a, C3-8 cycloalkyl, and C3-8 heterocycloalkyl, wherein the heterocycloalkyl groups are optionally substituted with 1-4 R2c groups; alternatively, two R2 groups linked to the same carbon are combined to form an oxo group (═O); alternatively, two R2 groups are combined to form a heterocycloalkyl ring having from 5 to 6 ring members and from 1 to 3 heteroatoms each independently selected from the group consisting of N, O, and S, wherein the heterocycloalkyl ring is optionally substituted with 1-3 R2d groups; R2a and R2b are each independently selected from the group consisting of hydrogen and C1-6 alkyl; each R2c is independently selected from the group consisting of hydrogen, halogen, hydroxy, C1-6 alkoxy, C1-6 haloalkoxy, CN, and NR2aR2b; each R2d is independently selected from the group consisting of hydrogen and C1-6 alkyl, or two R2d groups attached to the same ring atom are combined to form (═O); R3 is selected from the group consisting of phenyl and pyridyl, each optionally substituted with 1-4 R3a groups; each R3a is independently selected from the group consisting of hydrogen, halogen, and C1-6 haloalkyl; and subscript n is an integer from 0 to 3; or salts and isomers thereof. In some cases, the octahydro fused azadecalin has the formula: wherein R1 is a heteroaryl ring having from 5 to 6 ring members and from 1 to 4 heteroatoms each independently selected from the group consisting of N, O, and S, optionally substituted with 1-4 groups each independently selected from R1a; each R1a is independently selected from the group consisting of hydrogen, C1-6 alkyl, halogen, C1-6 haloalkyl, C1-6 alkoxy, C1-6 haloalkoxy, N-oxide, and C3-8 cycloalkyl; ring J is selected from the group consisting of an aryl ring and a heteroaryl ring having from 5 to 6 ring members and from 1 to 4 heteroatoms each independently selected from the group consisting of N, O, and S; each R2 is independently selected from the group consisting of hydrogen, C1-6 alkyl, halogen, C1-6 haloalkyl, C1-6 alkoxy, C1-6 haloalkoxy, C1-6 alkyl-C1-6 alkoxy, CN, OH, NR2aR2b, C(O)R2a, C(O)OR2a, C(O)NR2aR2b, SR2a, S(O)R2a, S(O)2R2a, C3-8 cycloalkyl, and C3-8 heterocycloalkyl having from 1 to 3 heteroatoms each independently selected from the group consisting of N, O, and S; alternatively, the two R2 groups on adjacent ring atoms are combined to form a heterocycloalkyl ring having from 5 to 6 ring members and from 1 to 3 heteroatoms each independently selected from the group consisting of N, O, and S, wherein the heterocycloalkyl ring is optionally substituted with 1-3 R2c groups; R2a, R2b, and R2c are each independently selected from the group consisting of hydrogen and C1-6 alkyl; each R3a is independently halogen; and subscript n is an integer from 0 to 3, or salts and isomers thereof. In yet another aspect, provided herein is a diagnostic composition, or a diagnostic kit comprising a glucocorticoid receptor antagonist (GRA) for use in a method of differentially diagnosing adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome in a patient where the differential diagnosis is between ectopic ACTH syndrome and Cushing Disease, the method comprising the step of determining the ACTH concentration ratio from a patient with Cushing's syndrome and an elevated ACTH level, where the patient has been administered a dose of glucocorticoid receptor antagonist (GRA) at least two hours prior to the removal of venous samples and where the amount of GRA administered to the patient is sufficient to increase ACTH from the pituitary gland by at least two fold in persons with normal Hypothalmus Pituitary Adrenal (HPA) function; wherein the ACTH concentration ratio is derived from the ACTH concentrations in fluid obtained from either the left or right inferior petrosal venous sinus and from fluid obtained from a periphery venous sample; and wherein an ACTH concentration ratio of greater than 3 for the ACTH concentration from the inferior venous sinus sample over the periphery venous sinus sample is diagnostic indicative of Cushing's disease. In addition, all the embodiments in the first aspect described above are also included in this aspect of the disclosure. In yet another aspect, provided here in is a method of obtaining a measurement indicative of differential diagnosis of adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome in a patient where the differential diagnosis is between ectopic ACTH syndrome and Cushing Disease, the method comprising the step of: (i) determining the ACTH concentration ratio from a patient with Cushing's syndrome and an elevated ACTH level, where the patient has been administered a dose of glucocorticoid receptor antagonist (GRA) at least two hours prior to the removal of venous samples and where the amount of GRA administered to the patient is sufficient to increase ACTH from the pituitary gland by at least two fold in persons with norma Hypothalmus Pituitary Adrenal (HPA) function; wherein the ACTH concentration ratio is derived from the ACTH concentrations in fluid obtained from either the left or right inferior petrosal venous sinus and from fluid obtained from a periphery venous sample; and wherein an ACTH concentration ratio of greater than 3 for the ACTH concentration from the inferior venous sinus sample over the periphery venous sinus sample is indicative of Cushing's disease. In addition, all the embodiments in the first aspect described above are also included in this aspect of the disclosure. In yet another aspect, provided herein is a glucocorticoid receptor antagonist (GRA) for use in a method of differentially diagnosing adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome in a patient where the differential diagnosis is between ectopic ACTH syndrome and Cushing Disease, the method comprising the steps of: (i) selecting a patient with Cushing's syndrome and also elevated ACTH levels; (ii) administering a dose of the GRA sufficient to increase ACTH from the pituitary gland by at least two fold in persons with normal Hypothalmus Pituitary Adrenal (HPA) function; (iii) waiting for at least two hours; and (iv) obtaining from the patient an ACTH concentration ratio wherein the ratio is derived from the ACTH concentrations in fluid obtained from either the left or right inferior petrosal venous sinus and from fluid obtained from a periphery venous sample; wherein an ACTH concentration ratio of greater than 3 for the ACTH concentration from the inferior venous sinus sample over the periphery venous sinus sample is diagnostic of Cushing's disease. In addition, all the embodiments in the first aspect described above are also included in this aspect of the disclosure. Other objects, features, and advantages of the present invention will be apparent to one of skill in the art from the following detailed description and FIGURES. Part DETAILED DESCRIPTION OF THE INVENTION: Part I. Introduction: This invention involves the use of GRAs to provide a robust and reproducible means to stimulate ACTH production in the pituitary gland for the differential diagnosis of patients with ACTH-dependent Cushing's syndrome, where the differential diagnosis is between ectopic ACTH syndrome and Cushing Disease. GRAs are first administered, and blood samples are then taken by IPSS after sufficient time for the assessment of ACTH levels. The claimed methods have many advantages over the existing differential diagnosis methods, such as CRH-IPSS, DDAVP-IPSS and metyrapone-IPSS. First, the claimed methods are more robust compared to metyrapone-IPSS. GRAs used in the invention act to block cortisol binding to the receptor—thus preventing cortisol from inhibiting ACTH production and resulting in increased ACTH production/secretion. Compared to metyrapone, which acts to block the cortisol synthesis pathway, GRAs' effect on ACTH stimulation is more direct, thus making the test results more reliable. Second, compared to CRH/DDAVP-IPSS, the methods are cost-effective and convenient to use because GRAs are orally deliverable and less expensive than CRH to manufacture and store. Third, compared to CRH/DDAVP-IPSS, the method disclosed herein dispenses with the need to sample blood before the administration of GRAs, and thus reduces the duration of catheterization and minimizes complications associated with prolonged catheterization. Part II. Definitions: The term “endogenous Cushing's syndrome” refers to a form of Cushing's syndrome, where the excess cortisol level is caused by the body's own overproduction of cortisol. The term “Adrenocorticotropic hormone (ACTH)-dependent Cushings syndrome” refers to a form of endogenous Cushing's syndrome, which is caused by abnormal production of ACTH. There are two major forms of ACTH-dependent Cushing's syndrome: Cushing Disease (accounting for about 80% of the cases) and ectopic ACTH syndrome (accounting for 20% of the cases). The term “ACTH concentration ratio”, “ACTH ratio”, “pituitary to periphery ACTH ratio”, or “central to periphery ACTH ratio” disclosed herein refers to the ratio between the amount, level, or concentration of ACTH in the blood sample obtained from inferior petrosal sinus and the blood sample obtained from the periphery veins. In one embodiment, the periphery vein is the jugular vein. The term “prolactin concentration ratio”, “prolactin ratio”, “pituitary to periphery prolactin ratio”, or “central to periphery prolactin ratio” disclosed herein refers to the ratio between the amount, level, or concentration of prolactin in the blood sample obtained from inferior petrosal sinus and the blood sample obtained from the periphery veins. In one embodiment, the periphery vein is the jugular vein. The term “differentially diagnosing” refers to the distinguishing of a particular disease or condition from others that present similar symptoms. A differential diagnostic method is a systematic diagnostic method used to identify the presence of a condition where multiple alternatives are possible. This method is essentially a process of elimination or a process of obtaining information that shrinks the “probabilities” of candidate conditions to negligible levels. The method uses evidence such as symptoms, test results, patient history, and medical knowledge to adjust epistemic confidences in the mind of the diagnostician (or, for computerized or computer-assisted diagnosis, the software of the system). Often each individual option of a possible disease is called a differential diagnosis. The term “ectopic ACTH syndrome” refers to the abnormal production of ACTH due to ectopic ACTH secretion by an extrapituitary tumor. These extrapituitary tumors frequently originate in lungs, but in some cases originate from the thymus, pancreas, adrenal gland or thyroid. The term “Cushing Disease” refers to the condition in which the pituitary gland releases too much ACTH as a result of a tumor located in—or excess growth (hyperplasia) of—the pituitary gland. Cushing Disease is a form of Cushing's syndrome. The term “hypercortisolemia” refers a condition of having a higher than normal amount of circulating cortisol. The term “inferior petrosal sinus sampling (IPSS)” refers to an invasive procedure performed to obtain blood samples from one or both petrosal venous sinuses by inserting catheters in one or both inferior petrosal veins via the jugular or femoral veins. The petrosal venous sinus drains the pituitary via the cavernous sinus. Thus, samples obtained from IPSS are often analyzed and compared with the samples obtained from periphery blood for the amount of a particular analyte to detect signs of a disease relating to the pituitary gland. The term “jugular venous sampling” refers to an invasive procedure performed to obtain blood samples from jugular veins (a periphery vein) by inserting catheters in the internal jugular vein via femoral veins. The tips of the catheters are typically advanced to the level of the angles of the mandible. The term “periphery venous sinus sampling” refers to an invasive procedure performed to obtain blood samples from periphery veins by catheterization. Non-limiting examples of periphery veins include adrenal veins, high inferior vena cava, hepatic vein, azygos and hemiazygos veins, right atrium, right and left innominate and thymic veins, jugular veins, and both superior and middle thyroid veins. The term “patient,” “individual”, or “subject” is used interchangeably to refer to a human subject. In some cases, the individual is suspected of having adrenal insufficiency. The term “administering” includes oral administration, topical contact, administration as a suppository, intravenous, intraperitoneal, intramuscular, intralesional, intrathecal, intranasal, or subcutaneous administration, or the implantation of a slow-release device, e.g., a mini-osmotic pump, to a subject. Administration is by any route, including parenteral and transmucosal (e.g., buccal, sublingual, palatal, gingival, nasal, vaginal, rectal, or transdermal). Parenteral administration includes, e.g., intravenous, intramuscular, intra-arteriole, intradermal, epicutaneous, subcutaneous, intraperitoneal, intraventricular, and intracranial. Other modes of delivery include, but are not limited to, the use of liposomal formulations, intravenous infusion, and transdermal patches. The term “sample” refers to a biological sample obtained from a human subject. The sample can be any cell, tissue or fluid from a human subject. Samples can be subject to various treatment, storage or processing procedures before being analyzed according to the methods described herein. Generally, the terms “sample” or “samples” are not intended to be limited by their source, origin, manner of procurement, treatment, processing, storage or analysis, or any modification. The term “cortisol” refers to a glucocorticoid hormone that is produced by the zona fasciculata of the adrenal gland. The term “adrenocorticotropic hormone” or “ACTH” refers to a polypeptide-based hormone that is normally produced and secreted by the anterior pituitary gland. ACTH stimulates secretion of cortisol and other glucocorticoids (GCs) by specialized cells of the adrenal cortex. In healthy mammals, ACTH secretion is tightly regulated. ACTH secretion is positively regulated by corticotropin releasing hormone (CRH), which is released by the hypothalamus. ACTH secretion is negatively regulated by cortisol and other glucocorticoids. The term “measuring the level,” in the context of cortisol, ACTH, or other steroids, refers determining, detecting, or quantitating the amount, level, or concentration of, for example, cortisol, ACTH or other steroids in a sample obtained from a subject. The term a “increase” or a “decrease” refers to a detectable positive or negative change in quantity from a comparison control, e.g., an established standard control (such as an average level of cortisol in a normal, healthy subject who does not have hypercortisolemia). An increase is a positive change that is typically at least 5%, at least 10%, or at least 20%, or 50%, or 100%, and can be as high as at least 1.5-fold, at least 2-fold, at least 5-fold, or even 10-fold of the control value. Similarly, a decrease is a negative change that is typically at least 5%, at least 10%, or at least 20%, 30%, or 50%, or even as high as at least 80% or 90% of the control value. Other terms indicating quantitative changes or differences from a comparative basis, such as “more,” “less,” ‘higher,” and “lower,” are used in this application in the same fashion as described above. The term “normal reference value”, “reference value”, or “standard control level” refers to the a predetermined amount, level, or concentration of a particular analyte, e.g., ACTH, cortisol, or prolactin—by comparison to which a diagnosis of the presence or absence of a particular condition can be made, e.g., hypercortisolemia. Normal reference values referred to in this disclosure are in some cases provided by the commercial test that is used to determine the analyte levels. In some cases, a normal reference value, reference value, or standard control level is established as the average of the amount, level, or concentration of an analyte from one or more normal, healthy subjects, e.g., subjects who have normal HPA function. In some cases, they are established as a range of the level, amount, or concentration of the analyte in a group of healthy subjects. Normal reference values may vary depending on the nature of the sample, the manner or timing of sample collection, as well as other factors such as the sex, age, and ethnicity of the subjects for whom such a control value is established. The term “elevated level”, “elevated amount”, or “elevated concentration” refers to the level or amount of the analyte that is higher than the normal reference value for that analyte. The term “chromatography” refers to a process in which a chemical mixture carried by a liquid or gas is separated into components as a result of the differential distribution of the chemical entities as they flow around or over a stationary liquid or solid phase. The term “liquid chromatography” or “LC” refers to a process of selective retardation of one or more components of a fluid solution when the fluid uniformly percolates either through a column of a finely divided substance or through capillary passageways. The retardation results from the distribution of the components of the mixture between one or more stationary phases and the bulk fluid, (i.e., mobile phase), as this fluid moves relative to the stationary phase(s). Examples of “liquid chromatography” include reverse phase liquid chromatography (RPLC), high performance liquid chromatography (HPLC), and turbulent flow liquid chromatography (TFLC) (sometimes known as high turbulence liquid chromatography (HTLC) or high throughput liquid chromatography). The term “high performance liquid chromatography” or “HPLC” (also sometimes known as “high pressure liquid chromatography”) refers to liquid chromatography in which the degree of separation is increased by forcing the mobile phase under pressure through a stationary phase—typically a densely packed column. As used herein, the term “ultra high performance liquid chromatography”, “HPLC” or “UHPLC” (sometimes known as “ultra high pressure liquid chromatography”) refers to HPLC which occurs at much higher pressures than in traditional HPLC techniques. The term “glucocorticosteroid” (“GC”) or “glucocorticoid” refers to a steroid hormone that binds to a glucocorticoid receptor. Glucocorticosteroids are typically characterized by having 21 carbon atoms, an α,β-unsaturated ketone in ring A, and an α-ketol group attached to ring D. They differ in the extent of oxygenation or hydroxylation at C-11, C-17, and C-19; see Rawn, “Biosynthesis and Transport of Membrane Lipids and Formation of Cholesterol Derivatives,” in Biochemistry, Daisy et al. (eds.), 1989, pg. 567. The term “glucocorticoid receptor” (“GR”) refers to the type II GR which specifically binds to cortisol and/or cortisol analogs such as dexamethasone; See, e.g., Turner & Muller, J Mol. Endocrinol, 2005 (35): 283-292. The GR is also referred to as the cortisol receptor. The term includes isoforms of GR, recombinant GR and mutated GR. Inhibition constants (Ki) against the human GR receptor type II (Genbank: P04150) are between 0.0001 nM and 1,000 nM; preferably between 0.0005 nM and 10 nM, and most preferably between 0.001 nM and 1 nM. The term “glucocorticoid receptor antagonist” or “GRA” refers to any composition or compound which partially or completely inhibits (antagonizes) the binding of a glucocorticoid receptor (GR) agonist, such as cortisol, or cortisol analogs, synthetic or natural, to a GR. A “specific glucocorticoid receptor antagonist” refers to any composition or compound which inhibits any biological response associated with the binding of a GR to an agonist. By “specific,” the drug preferentially binds to the GR rather than to other nuclear receptors, such as the mineralocorticoid receptor (MR), androgen receptor (AR), or progesterone receptor (PR). It is preferred that the specific glucocorticoid receptor antagonist binds GR with an affinity that is 10× greater ( 1/10th the Kd value) than its affinity to the MR, AR, or PR, both the MR and PR, both the MR and AR, both the AR and PR, or to the MR, AR, and PR. In a more preferred embodiment, the specific glucocorticoid receptor antagonist binds a GR with an affinity that is 100× greater ( 1/100th the Kd value) than its affinity to the MR, AR, or PR, both the MR and PR, both the MR and AR, both the AR and PR, or to the MR, AR, and PR. The term “selective inhibitor” in the context of a glucocorticoid receptor refers to a chemical compound that selectively interferes with the binding of a specific glucocorticoid receptor agonist and a glucocorticoid receptor. The term “steroidal backbone” in the context of glucocorticoid receptor antagonists containing such refers to glucocorticoid receptor antagonists that contain modifications of the basic structure of cortisol, an endogenous steroidal glucocorticoid receptor ligand. The basic structure of a steroidal backbone is provided as Formula I: The two most commonly known classes of structural modifications of the cortisol steroid backbone to create glucocorticoid antagonists include modifications of the 11-β hydroxy group and modification of the 17-β side chain (See, e. g., Lefebvre (1989) J. Steroid Biochem. 33: 557-563). As used herein, the term “non-steroidal backbone” in the context of glucocorticoid receptor antagonists containing such refers to glucocorticoid receptor antagonists that do not share structural homology to, or are not modifications of, cortisol. Such compounds include synthetic mimetics and analogs of proteins, including partially peptidic, pseudopeptidic, and non-peptidic molecular entities. Non-steroidal GRA compounds also include glucocorticoid receptor antagonists having a cyclohexyl-pyrimidine backbone, a fused azadecalin backbone, a heteroaryl ketone fused azadecalin backbone, or an octahydro fused azadecalin backbone. Exemplary glucocorticoid receptor antagonists having a cyclohexyl-pyrimidine backbone include those described in U.S. Pat. No. 8,685,973. Exemplary GRAs having a fused azadecalin backbone include those described in U.S. Pat. Nos. 7,928,237 and 8,461,172. Exemplary GRAs having a heteroaryl ketone fused azadecalin backbone include those described in U.S. Pat. Pub. 2014/0038926. Exemplary GRAs having an octohydro fused azadecalin backbone include those described in U.S. Provisional Patent Appl. No. 61/908,333, entitled Octahydro Fused Azadecalin Glucocorticoid Receptor Modulators, filed on Nov. 25, 2013. Where substituent groups are specified by their conventional chemical formulae, written from left to right, they equally encompass the chemically identical substituents that would result from writing the structure from right to left, e.g., —CH2O— is equivalent to —OCH2—. “Alkyl” refers to a straight or branched, saturated, aliphatic radical having the number of carbon atoms indicated. Alkyl can include any number of carbons, such as C1-2, C1-3, C1-4, C1-5, C1-6, C1-7, C1-8, C1-9, C1-10, C2-3, C2-4, C2-5, C2-6, C3-4, C3-5, C3-6, C4-5, C4-6, and C5-6. For example, C1-6 alkyl includes, but is not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, and hexyl. “Alkoxy” refers to an alkyl group having an oxygen atom that connects the alkyl group to the point of attachment: alkyl-O—. As for the alkyl group, alkoxy groups can have any suitable number of carbon atoms, such as C1-6. Alkoxy groups include, for example, methoxy, ethoxy, propoxy, iso-propoxy, butoxy, 2-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, pentoxy, hexoxy, etc. “Halogen” refers to fluorine, chlorine, bromine, and iodine. “Haloalkyl” refers to alkyl, as defined above, where some or all of the hydrogen atoms are replaced with halogen atoms. As for the alkyl group, haloalkyl groups can have any suitable number of carbon atoms, such as C1-6, and include trifluoromethyl, fluoromethyl, etc. The term “perfluoro” can be used to define a compound or radical where all the hydrogens are replaced with fluorine. For example, perfluoromethane includes 1,1,1-trifluoromethyl. “Haloalkoxy” refers to an alkoxy group where some or all of the hydrogen atoms are substituted with halogen atoms. As for the alkyl group, haloalkoxy groups can have any suitable number of carbon atoms, such as C1-6. The alkoxy groups can be substituted with 1, 2, 3, or more halogens. When all the hydrogens are replaced with a halogen, for example by fluorine, the compounds are per-substituted, for example, perfluorinated. Haloalkoxy includes, but is not limited to, trifluoromethoxy, 2,2,2,-trifluoroethoxy, and perfluoroethoxy. “Cycloalkyl” refers to a saturated or partially unsaturated, monocyclic, fused bicyclic, or bridged polycyclic ring assembly containing from 3 to 12 ring atoms, or the number of atoms indicated. Cycloalkyl can include any number of carbons, such as C3-6, C4-6, C5-6, C3-8, C4-8, C5-8, C6-8, C3-9, C3-10, C3-11, and C3-12. Saturated monocyclic cycloalkyl rings include, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cyclooctyl. Saturated bicyclic and polycyclic cycloalkyl rings include, for example, norbornane, [2.2.2]bicyclooctane, decahydronaphthalene, and adamantane. Cycloalkyl groups can also be partially unsaturated, having one or more double or triple bonds in the ring. Representative cycloalkyl groups that are partially unsaturated include, but are not limited to, cyclobutene, cyclopentene, cyclohexene, cyclohexadiene (1,3- and 1,4-isomers), cycloheptene, cycloheptadiene, cyclooctene, cyclooctadiene (1,3-, 1,4- and 1,5-isomers), norbornene, and norbornadiene. When cycloalkyl is a saturated monocyclic C3-8 cycloalkyl, exemplary groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. When cycloalkyl is a saturated monocyclic C3-6 cycloalkyl, exemplary groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. “Heterocycloalkyl” refers to a saturated ring system having from 3 to 12 ring members and from 1 to 4 heteroatoms of N, O, and S. Additional heteroatoms can also be useful, including but not limited to, B, Al, Si, and P. The heteroatoms can also be oxidized, such as, but not limited to, —S(O)— and —S(O)2—. Heterocycloalkyl groups can include any number of ring atoms, such as 3 to 6, 4 to 6, 5 to 6, 3 to 8, 4 to 8, 5 to 8, 6 to 8, 3 to 9, 3 to 10, 3 to 11, or 3 to 12 ring members. Any suitable number of heteroatoms can be included in the heterocycloalkyl groups, such as 1, 2, 3, or 4, or 1 to 2, 1 to 3, 1 to 4, 2 to 3, 2 to 4, or 3 to 4. The heterocycloalkyl group can include groups such as aziridine, azetidine, pyrrolidine, piperidine, azepane, azocane, quinuclidine, pyrazolidine, imidazolidine, piperazine (1,2-, 1,3- and 1,4-isomers), oxirane, oxetane, tetrahydrofuran, oxane (tetrahydropyran), oxepane, thiirane, thietane, thiolane (tetrahydrothiophene), thiane (tetrahydrothiopyran), oxazolidine, isoxalidine, thiazolidine, isothiazolidine, dioxolane, dithiolane, morpholine, thiomorpholine, dioxane, or dithiane. The heterocycloalkyl groups can also be fused to aromatic or non-aromatic ring systems to form members including, but not limited to, indoline. When heterocycloalkyl includes 3 to 8 ring members and 1 to 3 heteroatoms, representative members include, but are not limited to, pyrrolidine, piperidine, tetrahydrofuran, oxane, tetrahydrothiophene, thiane, pyrazolidine, imidazolidine, piperazine, oxazolidine, isoxazolidine, thiazolidine, isothiazolidine, morpholine, thiomorpholine, dioxane and dithiane. Heterocycloalkyl can also form a ring having 5 to 6 ring members and 1 to 2 heteroatoms, with representative members including, but not limited to, pyrrolidine, piperidine, tetrahydrofuran, tetrahydrothiophene, pyrazolidine, imidazolidine, piperazine, oxazolidine, isoxazolidine, thiazolidine, isothiazolidine, and morpholine. “Aryl” refers to an aromatic ring system having any suitable number of ring atoms and any suitable number of rings. Aryl groups can include any suitable number of ring atoms, such as 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 ring atoms, as well as from 6 to 10, 6 to 12, or 6 to 14 ring members. Aryl groups can be monocyclic, fused to form bicyclic or tricyclic groups, or linked by a bond to form a biaryl group. Representative aryl groups include phenyl, naphthyl and biphenyl. Other aryl groups include benzyl, that has a methylene linking group. Some aryl groups have from 6 to 12 ring members, such as phenyl, naphthyl, or biphenyl. Other aryl groups have from 6 to 10 ring members, such as phenyl or naphthyl. Some other aryl groups have 6 ring members, such as phenyl. Aryl groups can be substituted or unsubstituted. “Heteroaryl” refers to a monocyclic, fused bicyclic, or tricyclic aromatic ring assembly containing 5 to 16 ring atoms, where from 1 to 5 of the ring atoms are a heteroatom such as N, O, or S. Additional heteroatoms can also be useful, including but not limited to, B, Al, Si, and P. The heteroatoms can also be oxidized, such as, but not limited to, N-oxide, —S(O)—, and —S(O)2—. Heteroaryl groups can include any number of ring atoms, such as 3 to 6, 4 to 6, 5 to 6, 3 to 8, 4 to 8, 5 to 8, 6 to 8, 3 to 9, 3 to 10, 3 to 11, or 3 to 12 ring members. Any suitable number of heteroatoms can be included in the heteroaryl groups, such as 1, 2, 3, 4, or 5; or 1 to 2, 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, 2 to 5, 3 to 4, or 3 to 5. Heteroaryl groups can have from 5 to 8 ring members and from 1 to 4 heteroatoms, or from 5 to 8 ring members and from 1 to 3 heteroatoms, or from 5 to 6 ring members and from 1 to 4 heteroatoms, or from 5 to 6 ring members and from 1 to 3 heteroatoms. The heteroaryl group can include groups such as pyrrole, pyridine, imidazole, pyrazole, triazole, tetrazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-, 1,2,4-, and 1,3,5-isomers), thiophene, furan, thiazole, isothiazole, oxazole, and isoxazole. The heteroaryl groups can also be fused to aromatic ring systems, such as a phenyl ring, to form members including, but not limited to, benzopyrroles such as indole and isoindole, benzopyridines such as quinoline and isoquinoline, benzopyrazine (quinoxaline), benzopyrimidine (quinazoline), benzopyridazines such as phthalazine and cinnoline, benzothiophene, and benzofuran. Other heteroaryl groups include heteroaryl rings linked by a bond, such as bipyridine. Heteroaryl groups can be substituted or unsubstituted. The heteroaryl groups can be linked via any position on the ring. For example, pyrrole includes 1-, 2-, and 3-pyrrole; pyridine includes 2-, 3- and 4-pyridine; imidazole includes 1-, 2-, 4- and 5-imidazole; pyrazole includes 1-, 3-, 4- and 5-pyrazole; triazole includes 1-, 4- and 5-triazole; tetrazole includes 1- and 5-tetrazole; pyrimidine includes 2-, 4-, 5- and 6-pyrimidine; pyridazine includes 3- and 4-pyridazine; 1,2,3-triazine includes 4- and 5-triazine; 1,2,4-triazine includes 3-, 5- and 6-triazine; 1,3,5-triazine includes 2-triazine; thiophene includes 2- and 3-thiophene; furan includes 2- and 3-furan; thiazole includes 2-, 4- and 5-thiazole; isothiazole includes 3-, 4- and 5-isothiazole; oxazole includes 2-, 4- and 5-oxazole; isoxazole includes 3-, 4- and 5-isoxazole; indole includes 1-, 2- and 3-indole; isoindole includes 1- and 2-isoindole; quinoline includes 2-, 3- and 4-quinoline; isoquinoline includes 1-, 3- and 4-isoquinoline; quinazoline includes 2- and 4-quinoazoline; cinnoline includes 3- and 4-cinnoline; benzothiophene includes 2- and 3-benzothiophene; and benzofuran includes 2- and 3-benzofuran. Some heteroaryl groups include those having from 5 to 10 ring members and from 1 to 3 ring atoms including N, O, or S, such as pyrrole, pyridine, imidazole, pyrazole, triazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-, 1,2,4- and 1,3,5-isomers), thiophene, furan, thiazole, isothiazole, oxazole, isoxazole, indole, isoindole, quinoline, isoquinoline, quinoxaline, quinazoline, phthalazine, cinnoline, benzothiophene, and benzofuran. Other heteroaryl groups include those having from 5 to 8 ring members and from 1 to 3 heteroatoms, such as pyrrole, pyridine, imidazole, pyrazole, triazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-, 1,2,4- and 1,3,5-isomers), thiophene, furan, thiazole, isothiazole, oxazole, and isoxazole. Some other heteroaryl groups include those having from 9 to 12 ring members and from 1 to 3 heteroatoms, such as indole, isoindole, quinoline, isoquinoline, quinoxaline, quinazoline, phthalazine, cinnoline, benzothiophene, benzofuran and bipyridine. Still other heteroaryl groups include those having from 5 to 6 ring members and from 1 to 2 ring heteroatoms including N, O or S, such as pyrrole, pyridine, imidazole, pyrazole, pyrazine, pyrimidine, pyridazine, thiophene, furan, thiazole, isothiazole, oxazole, and isoxazole. Some heteroaryl groups include from 5 to 10 ring members and only nitrogen heteroatoms, such as pyrrole, pyridine, imidazole, pyrazole, triazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-, 1,2,4- and 1,3,5-isomers), indole, isoindole, quinoline, isoquinoline, quinoxaline, quinazoline, phthalazine, and cinnoline. Other heteroaryl groups include from 5 to 10 ring members and only oxygen heteroatoms, such as furan and benzofuran. Some other heteroaryl groups include from 5 to 10 ring members and only sulfur heteroatoms, such as thiophene and benzothiophene. Still other heteroaryl groups include from 5 to 10 ring members and at least two heteroatoms, such as imidazole, pyrazole, triazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-, 1,2,4- and 1,3,5-isomers), thiazole, isothiazole, oxazole, isoxazole, quinoxaline, quinazoline, phthalazine, and cinnoline. “Heteroatoms” refers to O, S, or N. “Salt” refers to acid or base salts of the compounds used in the methods of the present invention. Illustrative examples of pharmaceutically-acceptable salts are mineral acid (hydrochloric acid, hydrobromic acid, phosphoric acid, and the like) salts, organic acid (acetic acid, propionic acid, glutamic acid, citric acid, and the like) salts, and quaternary ammonium (methyl iodide, ethyl iodide, and the like) salts. It is understood that the pharmaceutically-acceptable salts are non-toxic. Additional information on suitable pharmaceutically-acceptable salts can be found in Remington's Pharmaceutical Sciences, 17th ed., Mack Publishing Company, Easton, Pa., 1985, which is incorporated herein by reference. “Isomers” refers to compounds with the same chemical formula but which are structurally distinguishable. “Tautomer” refers to one of two or more structural isomers which exist in equilibrium and which are readily converted from one form to another. Descriptions of compounds of the present invention are limited by principles of chemical bonding known to those skilled in the art. Accordingly, where a group may be substituted by one or more of a number of substituents, such substitutions are selected so as to comply with principles of chemical bonding and to produce compounds which are not inherently unstable—and/or would be known to one of ordinary skill in the art as likely to be unstable under ambient conditions—such as aqueous, neutral, or physiological conditions. “Pharmaceutically-acceptable excipient” and “pharmaceutically-acceptable carrier” refer to a substance that aids the administration of an active agent to—and absorption by—a subject and can be included in the compositions of the present invention without causing a significant adverse toxicological effect on the patient. Non-limiting examples of pharmaceutically-acceptable excipients include water, NaCl, normal saline solutions, lactated Ringer's, normal sucrose, normal glucose, binders, fillers, disintegrants, lubricants, coatings, sweeteners, flavors and colors, and the like. One of ordinary skill in the art will recognize that other pharmaceutical excipients are useful in the present invention. Part III. Detailed Descriptions of Embodiments: A. Method For Differential Diagnosis of ACTH-Dependent Cushing's Syndrome 1. Selecting Patients Having ACTH-Dependent Cushing's Syndrome The methods disclosed herein is used to provide differential diagnosis between Cushing Disease and ectopic ACTH syndrome to patients who have already been diagnosed as having ACTH-dependent Cushing's syndrome. A diagnosis of ACTH-dependent Cushing's syndrome can be made based on observation of certain clinical symptoms, the detection of hypercortisolemia and elevated blood ACTH levels. a. Clinical Symptoms Eligible patients may exhibit one or more of the following symptoms: easy bruising; abdominal obesity and thin arms and legs; facial plethora; acne; proximal myopathy (or proximal muscle weakness); striae (especially if reddish purple and 1 cm wide); and thin skin. Patients may also frequently feel changes in mood; change in appetite, headaches; a chronic feeling of tiredness; osteoporosis; low potassium; diabetes, and high blood pressure; decreased concentration peripheral edema hypokalemia; decreased libido acne kidney stones; impaired memory (especially short term); and unusual infections. Females patients may have irregular menstruation, hirsutism, or female balding. Pediatric patients may have weight gain with decreasing growth velocity; abnormal genital virilization; short stature; and pseudoprecocious puberty or delayed puberty. The next step is to confirm these patients have hypercortisolemia. b Hypercortisolemia A diagnosis of hypercortisolemia requires the determination of the patient's circulating cortisol level. Various types of samples that can be used for this purpose, such as saliva, urine, whole blood, serum, and plasma. Samples may also be collected at different time during the day. In one approach, the patient's whole blood sample is collected and processed to collect serum, i.e., in the morning, e.g., at 8 am. or in the afternoon, e.g., at 4 pm. The collected serum sample is refrigerated or frozen within, e.g., 2 hours of collection. Analysis of the serum sample is performed in a timely fashion, e.g. within 7 days from sample collection. In another approach, the patient's cortisol levels are measured from his or her saliva samples. Salivary cortisol is in equilibrium with the free cortisol in blood circulation. Changes of cortisol levels in the bloodstream are paralleled, within minutes, by similar alterations in salivary cortisol concentrations, such that one can use the latter as a surrogate of the former. The commonly used saliva-based cortisol test is the midnight saliva test, which measures cortisol levels from saliva samples collected at between 11 pm and midnight. Intake of food or drink is prohibited at least 15 minutes prior to sample collection. Saliva samples are collected by keeping and rolling a swab in mouth for approximately 2 minutes. The saliva samples, ambient or refrigerated, are then sent to a laboratory for cortisol level determination in a timely fashion, e.g., within 7 days from sample collection. Methods for measuring cortisol levels are known to those in the art. Useful assays include immunoassays, e.g., competitive immunoassay, radioimmunoassay, immunofluorometric enzyme assay, and ELISA, competitive protein-binding assay and mass spectrometry, e.g., high-performance liquid chromatography/triple quadrupole-mass spectrometry (LC-MS/MS). Commercial kits for measuring cortisol in samples are available from Beckman-Coulter, Seimens, Roche Diagnostics, and the like. Non-limiting examples of cortisol tests are Mayo Clinic's SALCT, CORT, CORTU, and CINP tests; an ADVIA Centaur® Cortisol assay (Siemens Healthcare Global); ARCHITECT i2000SR cortisol (Abbott); Immulite® 2000 Cortisol assay (Siemans Healthcare Global; #L2KCO2), Vitros® ECi Cortisol assay (Ortho Clinical Diagnostics; #107 4053), and Elecsys® Cortisol Immunoassay (Roche Molecular Diagnostics; #11875116160). The patient's cortisol measurement is then compared with the normal reference value; a level higher than the normal reference value indicates the patient has hypercortisolemia. The normal reference values for cortisol levels vary depending on the nature of the samples, the manner and timing of sample collection (higher for samples collected in the morning and lower for samples collected at night), and the detection method. Thus, it is essential to interpret test results in the context of the appropriate normal reference values. Various commercial kits provide the normal reference values in testing protocols. For example, normal reference values for the Mayo Clinic's SALCT test that determines cortisol level in saliva is <100 ng/dL; a saliva cortisol level higher than 100 ng/dL is thus an indication of hypercortisolemia. After being diagnosed with hypercortisolemia, the patient is subject to additional tests to confirm the presence of Cushing's syndrome. c Cushing's Syndrome At least one, preferably two or more, of the following tests are performed to diagnose Cushing's syndrome: 1) dexamethasone suppression test, which documents a loss of feedback inhibition of cortisol on the hypothalamic-pituitary-adrenal (HPA) axis; 2) 24-hour Urine Free Cortisol test, which assesses cortisol secretion in a 24-hour period; and 3) midnight salivary cortisol, which evaluates the loss of normal diurnal variation in cortisol secretion. If two of the three tests show abnormal cortisol levels, the Cushing's syndrome is confirmed. The dexamethasone suppression test is typically used as a screen test for Cushing's syndrome. Dexamethasone is an exogenous steroid that binds glucocorticoid receptors in the anterior pituitary gland. When healthy individuals are treated with a low dose (1-2 mg) of dexamethasone, binding of dexamethasone to the glucocorticoid receptors provides negative feedback to the pituitary gland and results in suppression of ACTH secretion. The suppression of ACTH secretion, in turn, results in suppression of cortisol release and therefore a detectable decrease in cortisol level in circulation. In contrast, when patients having Cushing's syndrome are treated with a low dose of dexamethasone, no or little decrease in cortisol levels can be detected because of the excessive cortisol production associated with the
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