Dexamethasone Suppression Test

Aka: Dexamethasone Suppression Test, Low-Dose Dexamethasone Suppression Test

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II. Background

  1. Two stage testing described below will miss cases
  2. The low dose (1 mg) test is not adequately sensitive

III. Indications: Cushing's Disease Screening

  1. Second line test
  2. Start with 24-hour Urinary free cortisol level

IV. Mechanism

  1. Test measures Adrenal Gland response to ACTH
  2. ACTH should decrease in response to Dexamethasone
  3. Decreased ACTH should result in decreased Cortisol
  4. Failure for Cortisol to suppress suggests 2 causes
    1. Pituitary GlandACTH over-produced or
    2. Adrenal GlandCortisol over-produced

V. Protocol

  1. Step 1: Draw Serum Cortisol at 4 pm on day 1
  2. Step 2: Patient takes Dexamethasone 1 mg at 11 pm day 1
  3. Step 3: Draw Serum Cortisol at 8-9 am on day 2

VI. Interpretation (Based on Serum Cortisol on day 2, 8 am)

  1. AM Serum Cortisol <5 mcg/dl (especially if <1.8 mcg/dl)
    1. Normal response (Suppresses)
  2. AM Serum Cortisol >5 mcg/dl
    1. Cushing's Disease
    2. Repeating with Dexamethasone 8 mg determines site
      1. Adrenal Gland source will not suppress
      2. Pituitary source will suppress

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