Ectopic Intact PTH Secretion Causing Humoral Hypercalcemia of Malignancy

Background: Tumor-generated ectopic intact PTH is difficult to diagnose and should be suspected in patients with apparent primary hyperparathyroidism but with normal parathyroid glands. Clinical Case: A 72-year-old man presented with symptoms of hypercalcemia including generalized weakness, polyuria, and polydipsia. Initial labs were consistent with primary hyperparathyroidism: calcium 12.1 mg/dL (n 8.6–10.3 mg/dL, albumin-corrected 12.5 mg/dL), intact PTH (iPTH) 115.6 pg/mL (n 10–65 pg/mL), low normal 25-OH vitamin D (25 ng/mL, n 25–100 ng/mL), and relatively high normal 1,25 dihydroxyvitamin D (52 pg/mL, n 18–78 pg/mL). 24-hour urine calcium was 381 mg/day (n 100–300 mg/day) and PTHrP was 1.6 pmol/L (n <4.2 pmol/L). Neck ultrasound demonstrated a 0.5 x 1 cm hypoechoic mass near right thyroid inferior pole, though sestamibi SPECT/CT scan did not reveal scintographic evidence of a parathyroid adenoma. He underwent subtotal parathyroidectomy with largest excised gland weighing 0.262 grams. The left inferior parathyroid gland appeared normal intraoperatively, thus was clipped and left in place. PTH decreased from 194 pg/mL to 98 pg/mL postoperatively. Pathological examination revealed three normocellular parathyroid glands with enlargement of only the right superior gland. Venous sampling of the parathyroid vasculature failed to identify the source of autonomous iPTH post operatively. Due to refractory hypercalcemia, cinacalcet was initiated. However, hypercalcemia as high as 12.6 mg/dl and hyperparathyroidism to 672 pg/mL persisted despite dose escalation. He eventually received pamidronate with subsequent transition to denosumab due to declining renal function. A (68)Ga DOTATATE scan was performed to locate occult ectopic parathyroid, which reported multiple foci of presumed somatostatin receptor expression involving the liver and intra-abdominal lymph nodes without significant uptake in the neck concerning for metastatic disease. Liver lesion biopsy was consistent with pancreato-biliary adenocarcinoma. Surprisingly, the biopsy was negative for iPTH and neuroendocrine tumor markers on staining/immunohistochemistry. Given his poor prognosis and multiple comorbidities, the patient opted not to pursue any further workup or therapy for his malignancy. Conclusion: Occult malignancy should be suspected for a patient with persistent hyperparathyroidism after parathyroidectomy. Treatment of the malignancy may lead to an improvement in hypercalcemia and iPTH levels. Employment of iPTH mRNA testing or intra-abdominal venous sampling to prove ectopic iPTH secretion would be ideal, as iPTH staining could be falsely negative. Further testing was not completed as the patient declined further evaluation.