Cushing Disease Mimicking Ectopic Adrenocorticotropic Hormone-Secreting Disease.

BACKGROUND/OBJECTIVE: We present the case of a patient whose initial biochemical results suggested an ectopic adrenocorticotropic hormone (ACTH)-secreting tumor, with markedly increased serum ACTH levels, increased 24-hour urinary free cortisol levels, and hypokalemia. Although these features are more commonly associated with ectopic ACTH secretion, the patient was ultimately diagnosed with Cushing disease. CASE REPORT: A 46-year-old man presented with recurrent headaches and progressive skin darkening. He had hypertension and a body mass index of 28.4 kg/m2. Initial blood work revealed hypokalemia at 3.2 mEq/L (normal reference range, 3.5-5.1 mEq/L), and screening tests for Cushing syndrome were pathologic including a highly increased urinary free cortisol level (605 μg/24 h; normal reference range, 20.9-292 μg/24 h). The serum ACTH level was high (89 pg/mL; normal reference range, 0-45.4 pg/mL), and pituitary magnetic resonance imaging showed an 8-mm lesion in the pituitary gland. Transsphenoidal pituitary surgery was performed with subsequent postsurgical remission. DISCUSSION: Higher urinary and plasma cortisol levels are typical of ectopic ACTH-secreting tumors and likely cause hypokalemia and hypertension, which are encountered more frequently in patients with ectopic ACTH-secreting tumors. In the absence of a pituitary mass 1 cm or more in size, further testing to differentiate Cushing disease from an ectopic source is required. CONCLUSION: Highly increased serum ACTH and cortisol levels, hypokalemia, and hypertension occur more frequently in patients with ectopic ACTH-secreting tumors. However, we show that they may occur in Cushing disease, and therefore, it would be wrong to assume a diagnosis of an ectopic ACTH-secreting tumor based on these findings alone.