MON-340 Concomitant TIO and SIADH in a Patient with Neuroendocrine Tumor

83 year old male patient with remote history of bladder cancer post TURBT, who presented with generalized weakness, poor appetite, and decreased PO intake of one week duration. He was found to have hyponatremia with Na level of 120mmol/L. IV hydration failed to correct his sodium levels. Persistent hyponatremia in the setting of euvolemia and inappropriately elevated urine sodium was suggestive of SIADH (serum Na 126 mmol/l, urine Na 79mmol/l). Lab work was also significant for hypocalcemia (7.4 mg/dl corrected for albumin) and hypophosphatemia (0.9-1.3 mg/dl). PTH was 48.3pg/ml which was inappropriately normal in the setting of hypocalcemia. 25(OH) vitamin D and 1,25(OH)(2) Vitamin D levels were measured and were 41 ng/ml and 19.5 pg/ml (19.9-79.3 pg/ml) respectively. Patient was started on calcium and vitamin D replacement. Hypophosphatemia was treated with potassium phosphate supplementation (up to 2 grams a day) with persistently low levels (1.4 mg/dl). 24 hour urine collection was performed and showed urine calcium excretion of 200 mg/24 hrs with a concomitant serum calcium level of 8.2 mg/dl and urine Phosphorus excretion of 684 mg/24hrs with a concomitant serum phosphorus of 1.3 mg/dl (Patient excreted 0.72 g creatinine in 24 hrs which was slightly low). These results were consistent with renal phosphate wasting. The combination of hypocalcemia, hypophosphatemia, renal phosphate wasting along with normal PTH levels is consistent with Oncogenic Osteomalacia. CT scan of the chest abdomen pelvis showed a large mediastinal mass suggestive of primary malignancy in addition to multiple lesions in the liver and adrenal and axillary lymphadenopathy. Liver biopsy was positive for poorly differentiated Neuroendocrine Carcinoma with small cell features. FGF23 level was elevated to 242 RU/ml (Ref range < or = 180 RU/ml) consistent with TIO. Phosphorus level corrected after initiating chemotherapy, as well as 1,25(OH)(2) VitD which increased to 51.7 pg/ml. Serum sodium and calcium also normalized. FGF23 was checked again after completing 2 sessions of therapy and was down to 89 RU/ml. Discussion: We report a case of combined TIO and SIADH in the setting of neuroendocrine mediastinal tumor with small cell features. The patient demonstrated hypophosphatemia, hypocalcemia, and hyponatremia due to elevated FGF-23, decreased hydroxylation of 25 OH vitamin D, and inappropriate ADH secretion. All these findings corrected with chemotherapy.