16th September 2024

Investigations

High levels of cortisol can be identified in the saliva, blood, and urine. A 24-hour urinary free cortisol, a late-night salivary cortisol test and the low dose dexamethasone suppression test which should suppress cortisol so if it is high, it is in keeping with Cushing syndrome. After this is confirmed, further localisation tests need to be performed which include:

• Plasma ACTH and cortisol levels at 9am and midnight.
  • If ACTH is high the cause is ACTH dependent (exogenous steroid or adrenal tumour)
  • If ACTH is low the cause is ACTH independent (Cushing disease or ectopic ACTH secretion)
• High dose dexamethasone suppression test (if cortisol suppressed then it indicates Cushing disease/pituitary adenoma)
• CRH stimulation test (if cortisol rises then it indicates a pituitary source)
• Petrosal sinus sampling measures the difference in ACTH level found in the inferior petrosal sinus (where the pituitary gland drains) compared to the periphery

By using these tests, healthcare providers can accurately diagnose Cushing disease and determine its underlying cause.