SAT-757 A Dark Turn: Severe Vitamin D Deficiency Triggering Hypercalcemic Crisis

Abstract Disclosure: T. Adamson: None. D. Alsurakhi: None. A. Khawar: None. N. Gharaibeh: None. U.S. Khawaja: None. A. Kundel: None. Hypercalcemic crisis is a rare but potentially life-threatening condition characterized by elevations in calcium levels, typically above 14 mg/dL [3]. It is most often due to primary hyperparathyroidism (pHPT) or malignancy, with less common causes being vitamin D toxicity and parathyroid carcinoma (PC). Though PC has an incidence rate of only 1% in all pHPT cases, clinical features such as Ca levels ≥14 mg/dL and parathyroid hormone (PTH) greater than three to 10 times the upper limit of normal raise suspicion for malignancy [4,6,7]. Vit D deficiency can contribute to hypercalcemia by reducing intestinal calcium absorption, leading to secondary hyperparathyroidism [2]. The clinical presentation of a hypercalcemic crisis can range from altered mental status, severe dehydration and in extreme cases, arrhythmias, coma or multisystem organ failure leading to death. While it is common for pHPT to cause hypercalcemia, it is relatively rare for pHPT to present with Ca > 14 mg/dL [1,5,8]. We present the case of a 57-year-old female with no significant medical history who developed hypercalcemic crisis exacerbated by severe Vit D deficiency. She had a 2-week history of nausea, vomiting, polyuria, polydipsia, dizziness, dry skin, and impaired concentration. Labs showed Ca 22.6 mg/dL, ionized Ca 2.23 mmol/L, PTH 1381 pg/mL, Vit D 7 ng/mL. Given the presentation and data suggesting PC is more often associated with Ca ≥14 mg/dL and PTH ranging from 276-2500 pg/mL, PC was the primary working diagnosis [6,7]. SPECT/CT showed a 46 mm mass in the inferior right thyroid lobe. This was suspicious for a parathyroid tumor due to its selective delayed washout, a characteristic of hyperfunctioning parathyroid tissue. She underwent parathyroidectomy and pathology confirmed a 3.7 × 3.4 × 2.2 cm densely cellular parathyroid adenoma with fibrosis and cystic change without atypia or necrosis. Postoperatively, the patient's calcium level decreased to 10.7 mg/dL with symptom resolution. She was subsequently discharged on Vit D and calcium supplementation. This case highlights two key points: first, while PC was initially the leading concern, it remains an increasingly rare final diagnosis, as reaffirmed here [4]. Second, severe Vit D deficiency can amplify hypercalcemic crises in pHPT, contributing to extreme Ca levels (22.6 mg/dL). Despite common hesitation to replete Vit D in pHPT due to fears of worsening hypercalcemia, this case underscores the importance of assessing and correcting all contributing factors, including Vit D status, in patients presenting with hypercalcemic crisis. Proper Vit D management plays a crucial role in maintaining calcium homeostasis and preventing life-threatening complications from dysregulated calcium metabolism. Presentation: Saturday, July 12, 2025