A Rare Presentation of Renal Sarcoidosis with Severe, Recurrent Hypercalcemia Despite Low-normal Vitamin D Levels.

Renal sarcoidosis is an uncommon extrapulmonary manifestation, rarely presenting with severe hypercalcemia. We describe a diagnostically challenging case of a 56-year-old female with goblet cell adenocarcinoma of the appendix, evaluated for acute kidney injury (AKI) and hypercalcemia after abnormal labs at a routine oncology visit. Initial assessment suggested a parathyroid-independent etiology. Extensive malignancy workup, including imaging and tumor markers, failed to identify an oncologic process. Despite aggressive resuscitation and bisphosphonate therapy, recurrent hypercalcemia with AKI persisted. Low-normal 1,25-dihydroxyvitamin D, and suppressed PTH further complicated diagnosis. Definitive diagnosis was made via renal biopsy, revealing granulomatous interstitial nephritis consistent with renal sarcoidosis. Treatment with corticosteroids led to rapid improvement in calcium and renal function. This case highlights the diagnostic complexity of renal sarcoidosis, especially when presenting with hypercalcemia without systemic clues. The delayed diagnosis underscores the importance of a broad differential and interdisciplinary collaboration in evaluating unexplained hypercalcemia.