SUN-776 Beyond the Usual Suspects: A Reversible Cause of PTH-Mediated Hypocalcemia

Abstract Disclosure: A. Alappatt: None. P. Leandro Bezerra: None. A.C. Zuzek: None. M. Falciglia: None. Hypocalcemia is a common abnormality with a presentation ranging from no symptoms to life-threatening neuromuscular or cardiovascular conditions. We describe a patient in the emergency department (ED) with severe hypocalcemia and complex comorbidities. A 70-year-old female with CHF and GERD was sent to the ED after severe hypocalcemia was discovered. Six years ago, she underwent partial thyroidectomy with radiation and chemotherapy for diffuse large B cell lymphoma of the thyroid. In the ED she reported muscle cramps, with numbness and weakness of her feet. Exam findings included a healed anterior neck scar, no neuromuscular abnormalities, and a negative Chvostek sign. Corrected calcium initially was 5.1 mg/dL (8.6 - 10.3) and the following morning was 6.3 mg/dL with a PTH level of 73 pg/mL (12.0 - 88.0). Magnesium, phosphorus, and 25-OH vitamin D were 0.7 mg/dL (1.5 -2.5), 7.3 mg/dl (2.1 - 4.5) and 46.5 ng/mL (30 - 100), respectively. Calcium and magnesium normalized with treatment and the subsequent increase in PTH to 140 pg/mL led to the diagnosis of hypomagnesemia-induced hypocalcemia. Because the low magnesium was likely due to use of diuretics and proton pump inhibitor, omeprazole was replaced with famotidine. She was discharged with calcium and magnesium supplementation and follow-up with endocrinology. Common causes of hypocalcemia include hypoparathyroidism, vitamin D deficiency, certain medications, and kidney disease. In the case described, the patient presented with severe hypocalcemia and PTH that was inappropriately normal. In adults, the most common cause of acquired hypoparathyroidism is surgery of the thyroid, parathyroids, or head and neck cancer. This can be transient, permanent, or intermittent. While post-surgical hypoparathyroidism typically occurs directly after surgery and our patient had surgery many years prior, chart review showed she was treated for hypocalcemia of unclear etiology in the past. Radiation has also been implicated as a cause of hypoparathyroidism with an onset that would be consistent with the timing in this case. Although the epidemiology is unclear, radiation induced hypoparathyroidism is believed to be rare. Low magnesium can cause hypocalcemia mediated by PTH resistance and/or blunted PTH secretion. In the patient described, the rise in PTH after normalizing magnesium suggested that hypomagnesemia-mediated hypocalcemia was the likely diagnosis. In this scenario, assessing the patient in the ED with incomplete information led initially to the incorrect assumption that the inadequate PTH response to severe hypocalcemia was related to prior thyroid surgery and/or radiation. With the increasing focus on efficiency in healthcare, consultants are often pressed for evaluations in rushed settings with insufficient data. This case highlights the importance of a systematic and disciplined approach to the differential diagnosis. Presentation: Sunday, July 13, 2025