The dexamethasone suppression test is designed to diagnose

Diagnosis

Diagnosis is the identification of the nature and cause of anything. Diagnosis is used in many different disciplines with variations in the use of logics, analytics, and experience to determine the cause and effect relationships...

 and differentiate among the various types of Cushing's syndrome

Cushing's syndrome

Cushing's syndrome is a hormone disorder caused by high levels of cortisol in the blood. This can be caused by taking glucocorticoid drugs, or by tumors that produce cortisol or adrenocorticotropic hormone or CRH...

 and other hypercortisol states.

It has also been used in the research of depression.

Physiology

Dexamethasone

Dexamethasone

Dexamethasone is a potent synthetic member of the glucocorticoid class of steroid drugs. It acts as an anti-inflammatory and immunosuppressant...

 is an exogenous steroid that provides negative feedback to the pituitary to suppress the secretion of ACTH. This steroid is unable to pass the blood brain barrier which allows this test to assess a specific part of the hypothalamic-pituitary-adrenal axis

Hypothalamic-pituitary-adrenal axis

The hypothalamic-pituitary-adrenal axis , also known as thelimbic-hypothalamic-pituitary-adrenal axis and, occasionally, as the hypothalamic-pituitary-adrenal-gonadotropic axis, is a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland ,...

. Specifically, dexamethasone binds to glucocorticoid

Glucocorticoid

Glucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor , which is present in almost every vertebrate animal cell...

 receptors in the pituitary gland, which lies outside the blood brain barrier, resulting in regulatory modulation.

Interpretation

Low-dose and high-dose variations of the test exist.

The test is given at low (usually 1–2 mg) and high (8 mg) doses of dexamethasone and the levels of cortisol

Cortisol

Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...

 are measured to obtain the results.

A low dose dexamethasone suppresses cortisol in individuals with no pathology in endogenous cortisol production.
A high dose dexamethasone exerts negative retro-control on pituitary ACTH producing cells but not on ectopic ACTH producing cells or adrenal adenoma.

Low-dose

A normal result is decrease in cortisol levels upon administration of low-dose dexamethasone. Results indicative of Cushing's disease involve no change in cortisol on low-dose dexamethasone, but inhibition of cortisol on high-dose dexamethasone. If the cortisol levels are unchanged by low- and high-dose dexamethasone then other causes of Cushing's syndrome must be considered with further work-up necessary.

High-dose

After the high-dose dexamethasone, it may be possible to make further interpretations.

ACTH	Cortisol	Interpretation
undetectable or low	is not suppressed by high or low doses.	Primary Adrenal Cushing syndrome is likely.
normal to elevated	is not suppressed by low or high doses	Ectopic ACTH syndrome is likely. If an adrenal tumor is not apparent, a chest CT and abdominal CT is indicated to rule out a different tumor secreting ACTH.
elevated	is not suppressed by low doses, but is suppressed by high doses	Cushing's disease Cushing's disease Cushing's disease is a cause of Cushing's Syndrome characterised by increased secretion of adrenocorticotropic hormone from the anterior pituitary. This is most often as a result of a pituitary adenoma...  should be considered. A pituitary MRI would be needed to confirm.

Equivocal results should be followed by a CRH

Corticotropin-releasing hormone

Corticotropin-releasing hormone , originally named corticotropin-releasing factor , and also called corticoliberin, is a polypeptide hormone and neurotransmitter involved in the stress response...

 stimulation test

ACTH stimulation test

The ACTH stimulation test is a medical test usually ordered and interpreted by endocrinologists to assess the functioning of the adrenal glands stress response by measuring the adrenal response to adrenocorticotropic hormone...

, with inferior petrosal sinus

Inferior petrosal sinus

The inferior petrosal sinuses, within the human head, are beneath the brain and allow blood to drain from the center of the head.They drain on either side inferiorly from the cavernous sinus and join with the sigmoid sinus to form the internal jugular vein, which continues inferiorly to drain...

sampling.