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A Physiologic Approach to Diagnosis of the Cushing Syndrome

Hershel Raff, PhD; and James W. Findling, MD
[+] Article and Author Information

From St. Luke's Medical Center, Medical College of Wisconsin, Milwaukee, Wisconsin.


Potential Financial Conflicts of Interest: None disclosed.

Requests for Single Reprints: Hershel Raff, PhD, Endocrinology and Diabetes, St. Luke's Medical Center, 2801 W KK River Parkway, Suite 245, Milwaukee, WI 53215; e-mail, hraff@mcw.edu.

Current Author Addresses: Drs. Raff and Findling: Endocrinology, St. Luke's Medical Center, Medical College of Wisconsin, 2801 W KK River Parkway, Suite 245, Milwaukee WI 53215.


Ann Intern Med. 2003;138(12):980-991. doi:10.7326/0003-4819-138-12-200306170-00010
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The general organization of the hypothalamic–pituitary–adrenal axis (Figure 1) has been appreciated for almost half a century. However, many recently described new concepts have significant implications for the understanding of the pathophysiologic characteristics, diagnosis, and treatment of the Cushing syndrome.

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Figure 1.
The hypothalamic–pituitary–adrenal control system.CRHAVPACTH

Neural pathways into the paraventricular nucleus of the hypothalamus can be classified as “stress” inputs (for example, hypoglycemia) directly and through the hippocampus and daily rhythm input (circadian rhythm). These inputs result in an activation of parvocellular neurons that release corticotrophin-releasing factor ( ) and arginine vasopressin ( ) into the capillary plexus of the median eminence, which forms long portal veins. These drain into the anterior pituitary, where CRH and AVP influence the corticotrophs to increase release of adrenocorticotrophic hormone ( ). The hormone enters the systemic circulation and stimulates the adrenal cortex to increase cortisol production. Cortisol exerts its biological effects through the glucocorticoid receptor and appears in the saliva and the urine. The hypothalamic–pituitary–adrenal axis loop is completed through glucocorticoid-negative feedback exerted at the anterior pituitary, hypothalamus, and hippocampus. Diagnostic tests associated with each physiologic process are shown in pink. CBG = corticosteroid-binding globulin; CS = Cushing syndrome.

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Figure 2.
Production of adrenocorticotropic hormone (ACTH) in the anterior pituitary corticotroph cell.LPHNN-POCM(8)

The large protein proopiomelanocortin (POMC) is produced by transcription and translation of the POMC gene. Adrenocorticotropic hormone is then produced by posttranslational processing. Note that other products of POMC can be produced (for example, γ-lipotropic hormone [ ], -terminal POMC fragment [ ], and melanocyte-stimulating hormone [ ]). Ectopic ACTH-secreting tumors can perform the same processing but often produce large amounts of precursors (particularly pro-ACTH). Modified from Findling JW, Raff H. Ectopic ACTH. In: Mazzaferri EL, Samaan NA, eds. Endocrine Tumors. Cambridge, MA: Blackwell Scientific; 1993:554-66, with permission from Blackwell Science Ltd. .

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Figure 3.
The cortisol–cortisone shuttle.11-HSD211-HSD1(4)(19)

The effect of cortisol on the mineralocorticoid receptor (for example, in the kidney) is prevented by metabolism to inactive cortisone by the enzyme 11β-hydroxysteroid dehydrogenase type 2 ( β ). Cortisone can be reactivated to cortisol by the enzyme 11β-hydroxysteroid dehydrogenase type 1 ( β ). When cortisol level is very high (as in the Cushing syndrome), not all of the excess cortisol can be inactivated to cortisone, and the effects of mineralocorticoid excess (for example, hypertension and hypokalemia) can occur. Modified from Seckl JR, Walker BR. Minireview: 11β-hydroxysteroid dehydrogenase type 1—a tissue-specific amplifier of glucocorticoid action. Endocrinology. 2001;142 :1371-6, with permission from The Endocrine Society . CNS = central nervous system.

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Figure 4.
The diagnosis of endogenous cortisol excess (spontaneous Cushing syndrome).CRH

Measurement of late-night (11:00 p.m.) salivary cortisol level and measurement of 24-hour urine free cortisol level are the initial diagnostic tests of choice. The low-dose dexamethasone suppression test must be used with caution with a stringent cutoff for serum cortisol level. If results are consistently abnormal, the Cushing syndrome is established. If the results of screening tests are equivocal, a dexamethasone–corticotropin-releasing hormone ( ) test can be performed. The physiologic principles exploited by each test are shown in pink.

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Figure 5.
The differential diagnosis of the Cushing syndrome.ACTHCTMRICRH

Once the diagnosis is established (see Figure 4), measurement of a suppressed plasma level of adrenocorticotropic hormone ( ) identifies ACTH-independent (adrenal) Cushing syndrome. Adrenal computed tomography ( ) is then performed, and a more detailed analysis is needed to differentiate among the subtypes of adrenal Cushing syndrome. The most challenging problem is the differential diagnosis of ACTH-dependent Cushing syndrome. The high-dose dexamethasone suppression test is no longer recommended. If the results of magnetic resonance imaging ( ) of the pituitary are clearly abnormal, referral to a neurosurgeon is appropriate. If not, bilateral petrosal sinus sampling with administration of corticotropin-releasing hormone ( ) is performed. This method reliably distinguishes pituitary Cushing disease from occult ectopic ACTH syndrome. * For a more thorough discussion, see text.

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