The phenomenon is best understood by the relative insensitivity of pituitary adenomas to the inhibitory effects of glucocorticoid stimulation ( Raff and Findling, 2003).

The fact that patients with ACTH-independent Cushing syndrome and those with ectopic ACTH secretion fail to suppress cortisol production during such testing is intuitive however, the reason for the failure of patients with ACTH-producing pituitary adenomas (Cushing disease) to suppress cortisol secretion after low-dose dexamethasone administration is less obvious. It is important to note that the exogenous dexamethasone is not detected by the serum cortisol assay. A key element to understanding this test is that despite the “low-dose” designation, the dose of dexamethasone administered for this study corresponds to threefold to fourfold levels of physiologic glucocorticoid. In subjects without hypercortisolism, dexamethasone acts on the corticotropic cells of the anterior pituitary gland, suppresses ACTH production, and thereby results in a reduction of serum cortisol levels ( Findling and Raff, 2005). Tests such as random serum cortisol, plasma ACTH level, urinary 17-ketosteroid level, insulin tolerance testing, and loperamide testing have fallen out of favor and are no longer recommended ( Nieman et al., 2008).Īdministration of low-dose dexamethasone followed by measurement of serum cortisol the next morning probes the patient's glucocorticoid negative feedback system. The Evaluation of Adrenal Lesions in Urologic Practice section at the end of this chapter outlines practical algorithms for the metabolic evaluation of adrenal lesions. The physiologic principles underlying these tests are important to understand and are described in this section ( Findling and Raff, 2005). Second-line tests include the 2-day LD-DST and midnight plasma cortisol testing ( Arnaldi et al., 2003 Newell-Price et al., 2006 Nieman et al., 2008 Pivonello et al., 2008). Late-night salivary cortisol is also used. The sensitivity of the former test, however, may not be adequate for screening patients with incidentalomas ( Nieman et al., 2008).

The two evaluations performed most frequently are the 24-hour urinary free cortisol (UFC) evaluation and the overnight LD-DST.

When the diagnosis of Cushing syndrome is suspected, the clinician can choose among several laboratory studies.

Partin MD, PhD, in Campbell-Walsh-Wein Urology, 2021 Establishing the Diagnosis of Cushing Syndrome

Elevated adrenal levels