Abstract
BACKGROUND: Insulin induces intracellular shift of phosphorus, causing hypophosphatemia. This is mild, transient and quiescent in normal persons. Persistent hyperinsulinemia in patients with depleted phosphorus stores can cause profound symptomatic hypophosphatemia. Clinical Case: A 36yr old man with MEN 1 presented in follow-up. His original manifestation of MEN 1 was familial primary hyperparathyroidism (pHPT) for which he had parathyroidectomy in 2009. At one of his endocrine clinic visits, he was noted to have asymptomatic hypophosphatemia with serum phosphorus of 2.4 (2.7–4.5 mg/dL) and mildly elevated PTH of 89 (9–77 pg/mL). This was presumed to be due to mild persistent/recurrent pHPT, and he was advised to increase dietary phosphorus intake. About a year later, he presented to the emergency room (ER) with palpitations and marked asthenia. Serum phosphorus was 1.0. He was treated with intravenous phosphorus and discharged on oral phosphorus replacement with advice to increase his dietary phosphate intake. Despite this, he continued to have recurrent spells of dizziness, weakness, palpitations, and marked hypophosphatemia several requiring repeat ER visits. His degree of mild pHPT remained stable and imaging studies to identify a possible cause for tumor induced osteomalacia with phosphate wasting was unrevealing. He volunteered at a follow up visit that he had a long history of low blood sugar spells (confirmed by fingerstick) that he had learnt to manage with frequent carbohydrate rich snacks. This prompted the consideration of a possible insulinoma as the cause for his persistent hypophosphatemia. Despite significant hypoglycemia on an abbreviated supervised fast, an OGTT and mixed meal test showed profound severe insulin mediated hypoglycemia. CGMS also showed multiple extended early morning severe hypoglycemic spells <50mg/dl over a 2 wk period. Multiple imaging tests were negative but a Gallium Dotatate scan revealed multiple areas of uptake in his pancreas. Endoscopic ultrasound of the pancreas demonstrated multiple masses in the pancreas: one 7 x 5 cm mass in the body and one 4 x 4 cm mass near the neck of the pancreas. Fine needle aspiration biopsy revealed well-differentiated neuroendocrine tumors. He had distal pancreatectomy and tumor enucleation of 5 tumors identified on intraoperative sonography. Histopathology confirmed multiple neuroendocrine tumors, with immunostaining for insulin. He has had no further episodes of hypophosphatemia since despite being off oral phosphate supplements. Conclusion: Profound hypophosphatemia can be the dominant presentation of persistent hyperinsulinemia seen in patients with insulinoma. Insulinomas and other causes of persistent hyperinsulinemia need to be considered in the differential diagnosis of persistent/recurrent hypophosphatemia.