Lab

Dexamethasone Suppression Test

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Dexamethasone Suppression Test, Low-Dose Dexamethasone Suppression Test

  • Background
  1. Two stage testing described below will miss cases
  2. The low dose (1 mg) test is not adequately sensitive
  1. Second line test
  2. Start with 24-hour Urinary free cortisol level
  • 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 Gland ACTH over-produced or
    2. Adrenal Gland cortisol over-produced
  • 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
  • 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