Lab
Dexamethasone Suppression Test
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Dexamethasone Suppression Test
, Low-Dose Dexamethasone Suppression Test
Background
Two stage testing described below will miss cases
The low dose (1 mg) test is not adequately sensitive
Indications
Cushing's Disease
Screening
Second line test
Start with
24-hour Urinary free cortisol level
Mechanism
Test measures
Adrenal Gland
response to
ACTH
ACTH
should decrease in response to
Dexamethasone
Decreased
ACTH
should result in decreased
Cortisol
Failure for
Cortisol
to suppress suggests 2 causes
Pituitary Gland
ACTH
over-produced or
Adrenal Gland
Cortisol
over-produced
Protocol
Step 1: Draw
Serum Cortisol
at 4 pm on day 1
Step 2: Patient takes
Dexamethasone
1 mg at 11 pm day 1
Step 3: Draw
Serum Cortisol
at 8-9 am on day 2
Interpretation (Based on Serum Cortisol on day 2, 8 am)
AM
Serum Cortisol
<5 mcg/dl (especially if <1.8 mcg/dl)
Normal response (Suppresses)
AM
Serum Cortisol
>5 mcg/dl
Cushing's Disease
Repeating with
Dexamethasone
8 mg determines site
Adrenal Gland
source will not suppress
Pituitary source will suppress
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