Crossing the other side of the algorithm: a challenging case of adrenal Cushing's syndrome

The diagnosis of endogenous Cushing's syndrome and its aetiology involved documenting the hypercotisolism and then determining whether that hypercortisolism is adrenocorticotropic hormone-dependent (ACTH-dependent) or not. Hence, following the algorithm, an undetected ACTH level points to an adrenal Cushing's while a detectable or elevated ACTH level points to either a pituitary or ectopic Cushing's syndrome. The authors present a case of florid adrenal Cushing's syndrome initially presenting with a normal ACTH level, which led to the investigation for an ACTH-secreting tumour. Adding to the confusion, a MRI done showed an intrasellar focus. Knowledge of how ACTH-dependent (versus ACTH-independent) Cushing's syndrome manifests clinically, supported by results of repeat laboratory tests, led to the true diagnosis. This case illustrates that a detectable ACTH does not rule out an adrenal Cushing's syndrome nor does a positive pituitary imaging confirm Cushing's disease.