Lab

Dexamethasone Suppression Test

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Dexamethasone Suppression Test, Low-Dose Dexamethasone Suppression Test

  • See Also
  1. Cushing's Syndrome
  1. Preferred test if adrenal tumor suspected or Night Shift Worker (altered circadian rhythm)
  2. Among 3 first line options for Cushing's Syndrome Screening
    1. See 24-hour Urinary Free Cortisol level
    2. See Late-Night Salivary Cortisol (LNSC)
  • 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
  • Protocol
  1. Step 1: Patient takes Dexamethasone 1 mg at 11 pm on day 1
  2. Step 2: Draw Serum Cortisol at 8-9 am on day 2
  • Interpretation (Based on 8 am Serum Cortisol on day 2)
  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
  1. Increased gastrointestinal transit (e.g. Diarrhea)
  2. Pseudo-Cushing's Syndrome
  3. CYP3A4 Inducers (increase Dexamethasone metabolism)
    1. Neuropsychiatric drugs (Carbamazepine, Phenobarbital, Phenytoin,St. John's Wort)
    2. Rifampin
    3. Troglitazone
  4. Corticosteroid Binding Globulin increased
    1. Pregnancy
    2. Estrogen
    3. Chronic hepatitis
  1. Hepatic Impairment
  2. Renal Impairment (eGFR<60 ml/min)
  3. Hypoalbuminemia
  4. Corticosteroid Binding Globulin decreased
  5. CYP3A4 Inhibitors (decrease Dexamethasone metabolism)
    1. Cimetidine
    2. Ritonavir
    3. Azole Antifungals (Itraconazole, Fluconazole, Ketoconazole)
    4. Calcium Channel Blockers (Diltiazem, Nifedipine, Verapamil)
  • Efficacy
  1. Some protocols recommend a 3 stage Dexamethasone test for higer Test Sensitivity
    1. Draw Serum Cortisol at 4 pm on day 1 before Dexamethasone
    2. Accuracy is also improved when Dexamethasone level is checked on day 2 (in addition to Cortisol)
  2. Low-Dose Dexamethasone Suppression Test
    1. Test Sensitivity: 85 to 90%
    2. Test Specificity: 95 to 99%
    3. Strong Negative Predictive Value
  • References
  1. Elamin (2008) J Clin Endocrinol Metab 93(5):1553-62 +PMID: 18334594 [PubMed]
  2. Galm (2020) J Clin Endocrinol Metab 105(6):dgaa105 +PMID: 32133504 [PubMed]
  3. Maness (2024) Am Fam Physician 110(3): 270-80 [PubMed]