High-Dose Dexamethasone Suppression Tests

* basis: ACTH secretion in Cushing’s disease is not completely, but only relatively, resistant to glucocorticoid negative feedback inhibition.

Standard Two-Day Test

1. collect at least one baseline 24-h urine specimen, usually beginning at 8 AM.

2. After baseline collection is completed, the patient begins taking 2 mg dexamethasone orally every 6 h for a total of 8 doses, and the urine collections are continued.

# In practice, this test is often performed immediately after completing the low-dose dexamethasone suppression test and no intervening baseline urine collection is obtained.

3. The urine collections are assayed for 17-OHC, free cortisol, and creatinine levels.

4. In addition, a blood specimen can be collected 6 h after the last dose of dexamethasone for cortisol, dexamethasone, and ACTH radioimmunoassay.

Normal Values

* Urinary 17-OHCS : less than 6.9μmol (2.5 mg)/24 h

free cortisol: < 28 nmol/d (10μg/d)

Plasma cortisol and ACTH concentration are low and usually undetectable

Plasma dexamethasone concentrations: 31 to 41 nmol/L (12 to 16 ng/mL)

Overnight Test

1. Dexamethasone (8 mg) is taken orally between 11 PM and midnight, and a single blood sample is drawn at 8 AM the nest morning for assay of plasma cortiosl, and if one wishes, ACTH and dexamethasone.

Normal Values:

* 8 AM plasma cortisol level: < 140 nmol/L (5μg/dL)

Plasma ACTH level: 20 to 61 nmol/L (8 to 24 ng/mL)

Interpretation

* significant suppression of urinary 17-OHCS and free cortisol: Cushing’s disease, bronchial carcinoid tumors