Serum cortisol levels during circadian studies and high and low dexamethasone suppression tests.
In a healthy individual, the serum cortisol level falls naturally during the day, and administration of a low dose of oral dexamethasone (0.5mg six-hourly) will lead to suppression of cortisol levels below 50nmol/l. No fall occurs in patients with ectopic ACTH syndrome, even with high doses of dexamethasone. This contrasts with the responses seen in patients with pituitary-dependent disease in whom inadequate suppression after low-dose administration of dexamethasone, and at least fifty per cent suppression after administration of a high dose of dexamethasone (2mg six-hourly), are characteristic.
