Differential Diagnosis for Elevated Serum Cortisol with ACTH >15, Failed LDDST, and Partially Empty Sella

• Single Most Likely Diagnosis
  • Cushing's disease: This is the most likely diagnosis given the elevated serum cortisol levels, high ACTH levels (>15), and the failure of the low-dose dexamethasone suppression test (LDDST). The partially empty sella could be related to a pituitary adenoma causing Cushing's disease.
• Other Likely Diagnoses
  • Ectopic ACTH-producing tumor: Although less common than Cushing's disease, an ectopic ACTH-producing tumor could explain the elevated cortisol and ACTH levels. However, these tumors typically do not respond to the high-dose dexamethasone suppression test, which was not mentioned.
  • Familial Cushing's syndrome: This is a rare genetic condition but could be considered if there's a family history of Cushing's syndrome.
• Do Not Miss Diagnoses
  • Pheochromocytoma: Although primarily associated with hypertension and catecholamine excess, some pheochromocytomas can co-secrete ACTH, leading to Cushing's syndrome. Missing this diagnosis could be deadly due to the potential for hypertensive crises.
  • Nelson's syndrome: This condition occurs in patients who have had both adrenal glands removed (bilateral adrenalectomy) for Cushing's disease and then develop an aggressive pituitary tumor. It's crucial to consider in patients with a history of Cushing's disease treatment.
• Rare Diagnoses
  • McCune-Albright syndrome: A genetic disorder that can cause Cushing's syndrome among other symptoms, due to adrenal or pituitary abnormalities.
  • Primary pigmented nodular adrenocortical disease (PPNAD): A rare cause of Cushing's syndrome, often associated with Carney complex, characterized by multiple small nodules in the adrenal glands.