Dual paraneoplastic syndrome with prostate cancer: A case of syndrome of inappropriate secretion of antidiuretic hormone (SIADH) and humoral hypercalcemia of malignancy (HHM).

We report the case of an 85-year-old man diagnosed with metastatic prostate cancer three years earlier. While on combined androgen blockade, he developed hematuria, hematochezia, and fatigue. These symptoms were attributed to local progression of prostate cancer and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Water restriction, palliative radiation, and tolvaptan alleviated the symptoms. Four months later, he experienced appetite loss and fatigue due to hypercalcemia, ultimately leading to a diagnosis of humoral hypercalcemia of malignancy (HHM).