Indications for the procedure

To differentiate between pituitary-dependent and ectopic causes of Cushing’s syndrome.

Preparations for the test

No special preparations are required.

Drug administration

Drug(s) Given

Dexamethasone tablets available as 500 microgram tablets.
Give 2mg every 6 hours for as total of eight doses.
In younger children give 80 micrograms/kg/dose every 6 hours for a total of eight doses.
Maximum single dose is 2mg.

How Given

Orally on day two of the test.

Timing of Administration

After basal urine and blood samples have been taken.

Procedure

Time Action
Day 1 @ 09:00 Take venous blood sample. Start 24 hour urine collection.
Day 1 @ midnight Take venous blood sample.
Day 2 @ 06:00, 12:00, 18:00, 00:00 Take oral dexamethasone 6 hourly
Day 3 @ 06:00, 12:00, 18:00, 00:00 Take oral dexamethasone 6 hourly
Day 4 @ 06:00 Take venous blood sample. Start 24 hour urine collection.

Samples required

Time      
Day 1 @ 09:00 Cortisol ACTH Urine steroids
Day 1 @ midnight Cortisol ACTH  
Day 4 @ 06:00 Cortisol ACTH Urine steroids

Interpretation

In Pituitary-dependent hypercortisolism (Cushing’s disease), the plasma cortisol concentration usually suppresses to at least 50% of basal values.

In approximately 10% of patients with Cushing’s disease there is a failure to suppress cortisol.

Approximately 10% of patients with ectopic ACTH secretion will suppress.

It is important to discuss the use of dexamethasone suppression and the interpretation of the results with a paediatric endocrinologist.