Cushing's Syndrome due to a Renal Neuroendocrine Tumor: A Case Report.

Introduction: Cushing's syndrome (CS) due to ectopic adrenocorticotrophic hormone (ACTH) is rare and usually due to neuroendocrine neoplasia (NEN). Primary renal NEN is exceptionally rare but may be a cause of rapidly progressive CS.

Case presentation: A 51-year-old man presented with profound hypokalemia, cellulitis, and new-onset type 2 diabetes and hypertension with 1 month of muscle weakness, labile mood, and insomnia. CS due to ectopic ACTH production was confirmed. Biochemical control was achieved using a "block-and-replace" regimen with dual blockade with ketoconazole and metyrapone and hydrocortisone replacement in addition to mineralocorticoid receptor blockade using spironolactone. CT and ultrasound demonstrated a 24 mm right renal lesion with features concerning for renal cell carcinoma. Right laparoscopic nephrectomy was performed. Histology demonstrated a WHO grade one NEN with ACTH staining.

Conclusion: In CS, where the source of ectopic ACTH production is unable to be identified, a renal source should be considered. Diagnosis may be difficult as there are no reliable radiological characteristics to distinguish renal NENs from renal cell carcinomas, so a high degree of suspicion is required.