Cushing's Disease - Diagnosis

Diagnosis

Diagnosis is made first by diagnosis of Cushing's Syndrome which can include salivary and blood serum cortisol testing, 24-hour urinary free cortisol testing, and the dexamethasone suppression test.

Following the 8 mg dexamethasone suppression test plasma cortisol should be low; while plasma ACTH should be at the higher end of normal or should exceed the reference range. This will distinguish whether the cause is an ectopic ACTH secreting tumour, an adrenal carcinoma or a pituitary adenoma - ranges differ between different labs. A more accurate but invasive test used to differentiate pituitary from ectopic or adrenal cushing's syndrome is inferior petrosal sinus sampling.

A CT or MRI of the pituitary may also show the ACTH secreting tumor if present.

Administration of corticotropin releasing hormone (CRH) can differentiate this condition from ectopic ACTH secretion. If ectopic, the plasma ACTH and cortisol levels should remain unchanged; if this is pituitary related, levels of both would rise.