Dexamethasone suppression test

Dexamethasone suppression test
Diagnostics

Dexamethasone
OPS-301 code 1-797
proopiomelanocortin derivatives
POMC
     
γ-MSH ACTH β-lipotropin
         
  α-MSH CLIP γ-lipotropin β-endorphin
       
    β-MSH  

The dexamethasone suppression test is designed to diagnose and differentiate among the various types of Cushing's syndrome and other hypercortisol states.[1]

It has also been used in the research of depression.[2]

Contents

Physiology

Dexamethasone 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. Specifically, dexamethasone binds to glucocorticoid receptors in the pituitary gland, which lies outside the blood brain barrier, resulting in regulatory modulation.[3]

Interpretation

Low-dose and high-dose variations of the test exist.[4]

The test is given at low (usually 1–2 mg) and high (8 mg) doses of dexamethasone and the levels of cortisol are measured to obtain the results.[1]

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.[5]

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 should be considered. A pituitary MRI would be needed to confirm.

Equivocal results should be followed by a CRH stimulation test, with inferior petrosal sinus sampling.

References