Table 5

 Identification of the source of hypercortisolaemia

ACTH, adrenocorticotrophic hormone; CRH, corticotrophin releasing hormone; MRI, magnetic resonance imaging.
• Dexamethasone suppression: any degree of suppression of circulating cortisol in response to dexamethasone suggests pituitary dependent disease
• ACTH: high values suggest ectopic ACTH secretion (such as lung or pancreatic carcinoids) and undetectable ACTH suggests adrenal dependent disease; in pituitary dependent disease values are often normal or moderately elevated, but may be undetectable
• MRI: incidental pituitary lesions ⩽3 mm in diameter are present in almost 10% of the general population; conversely, pituitary MRI is normal in 20% of proven Cushing’s disease—hence the exhortation “do not use a localising procedure before you’ve carried out a diagnostic test”
• Inferior petrosal sinus sampling with CRH stimulation: a central to peripheral ACTH ratio of more than 2, increasing to more than 3 following a peripheral CRH bolus, is highly specific for pituitary dependent disease