SPEG (Scottish Paediatric Endocrine Group)
Diagnosis of Cushing’s syndrome.
Low dose dexamethasone suppression test
A 24-hour urine collection for urinary free cortisol and urinary steroid profiles should be taken before dexamethasone is given.
Drug(s) Given
Dexamethasone 0.5 mg every 6 hours for two days total of eight doses)
In younger children give 20 micrograms/kg/dose every 6 hours.
Maximum single dose is 0.5 mg.
How Given
Orally in tablet form.
Timing of Administration
Start oral dexamethasone at on the morning following completion of urine collection (day two).
| Time | Action |
| Day 1 @ 09:00 | Take venous blood sample Start urine collection. |
| Day 1 @ midnight | Take venous sample. |
| Day 2 @ 06:00, 12:00, 18:00, 00:00 | Give dexamethasone 6 hourly. |
| Day 3 @ 06:00, 12:00, 18:00, 00:00 | Give dexamethasone 6 hourly. |
| Day 4 @ 06:00 | Take venous sample. Start urine collection. |
| Time | |||
| Day 1 @ 09:00 | Cortisol | ACTH | Urine steroids |
| Day 1 @ midnight | Cortisol | ACTH | |
| Day 4 @ 06:00 | Cortisol | ACTH | Urine steroids |
In normal circumstances, Cortisol concentrations should suppress to less than 50nmol/L.
In patients with Cushing’s syndrome from whatever cause, there is loss of the normal negative feedback control by circulating glucocorticoids on ACTH release.
These patients will exhibit detectable plasma ACTH and cortisol concentrations after dexamethasone administration.
In patients who fail to show cortisol suppression, a pre-test ACTH level of less than 5ng/L is highly suggestive of an adrenal cause of Cushing’s syndrome.