Ectopic adrenocorticotropic hormone (ACTH) is a rare cause of Cushing’s syndrome, accounting for approximately 5 to 10 % of cases (1). The optimal treatment of ectopic ACTH is surgical resection of the underlying tumour (1).
We present a challenging case of ectopic ACTH in a 74-year-old male with a rapid onset of signs and symptoms. He had a background of frequent and prolonged exogenous steroid use. Initial biochemical testing revealed a significantly elevated low-dose dexamethasone suppression test, 24-hour urinary free cortisol, midnight salivary cortisol, and ACTH. Neuroimaging revealed a 3 mm pituitary adenoma. Dynamic testing with a corticotrophin-releasing hormone stimulation test and high-dose dexamethasone test were suggestive of ectopic ACTH over pituitary disease. Computed tomography (CT) did not identify a source. During this time, the patient had multiple admissions to hospital with worsening glycemic control and hypertension, progressive proximal myopathy, as well as hypokalaemia and sepsis. He was subsequently commenced on medical therapy and eventually had a 68Ga-DOTA-somatostatin analogue positron emission tomography (PET)/CT scan which did not demonstrate any identifiable focus for ectopic ACTH production. The patient declined to have bilateral adrenalectomy, and chose to remain on medical therapy with ongoing surveillance.
We faced many diagnostic challenges with this patient. The history of exogenous steroid use and the finding of a small pituitary adenoma were both red-herrings. We were also unable to localise the source of ectopic ACTH. Conventional imaging studies, such as CT, can localise the tumour in approximately 50 % of cases (2). 68Ga-DOTA-somatostatin analogue PET/CT has the highest sensitivity for localising covert ACTH-producing tumours (2). Medical therapy can be continued for prolonged periods in patients with covert disease, allowing serial imaging over time to identify the tumour (1). However, in the case of indolent disease, it can take years to decades before the source is identified (1).