Endogenous Cushing syndrome (CS) is an uncommon disease in which patients often present with nonspecific initial symptoms that may affect many organ systems, with considerable morbidity and mortality. In the diagnosis of CS, the use of appropriate laboratory tests for the initial documentation of hypercortisolism as well as for distinguishing between adrenocorticotropic hormone dependent and independent forms are important for identifying the source and localizing the lesion. These tests are also essential for guiding the clinician to the correct surgical procedure to potentially cure the patient.