Giant Parathyroid Adenoma Masked by Severe Vitamin D Deficiency with Hypocalcemic Presentation: A Case Report.

Introduction: Giant parathyroid adenomas, defined as lesions weighing at least 3.5 grams, are rare and typically present with hypercalcemia due to excessive parathyroid hormone secretion. Severe vitamin D deficiency can alter this pattern by lowering serum calcium, which may obscure the diagnosis of primary hyperparathyroidism, especially in younger patients. This case is notable because a large parathyroid adenoma presented with hypocalcemia rather than hypercalcemia, creating an atypical biochemical profile that initially mimicked secondary hyperparathyroidism. Case Presentation: A 30-year-old Syrian woman presented with a nine-month history of progressive fatigue, generalized bone pain, intermittent paresthesia in her fingertips, and cervical discomfort. Physical examination was unremarkable, with no palpable cervical or thyroid mass. Laboratory evaluation demonstrated severe vitamin D deficiency, hypocalcemia with low total and ionized calcium, and markedly elevated parathyroid hormone levels. She was started on calcium and vitamin D supplementation, which normalized calcium levels; however, parathyroid hormone levels remained inappropriately elevated, thereby unmasking underlying autonomous primary hyperparathyroidism. Two ultrasound examinations were non-diagnostic, whereas 99mTc-sestamibi scintigraphy showed persistent radiotracer uptake consistent with a hyperfunctioning left inferior parathyroid gland. The patient underwent focused parathyroidectomy, and a 3.5-gram adenoma was excised. Histopathological examination confirmed a benign giant adenoma without capsular or vascular invasion. Postoperatively, serum calcium remained stable and parathyroid hormone levels were appropriately suppressed. The patient experienced no symptoms of hypocalcemia and did not develop hungry bone syndrome. She was discharged the same day and had an uneventful recovery with normal biochemical findings on follow-up. Conclusion: This case demonstrates how severe vitamin D deficiency can mask the expected biochemical features of primary hyperparathyroidism and delay recognition of a giant parathyroid adenoma. It highlights the limitations of ultrasound in detecting deep or inferior parathyroid glands and emphasizes the importance of functional imaging when anatomic studies are inconclusive. Recognition of this atypical presentation is essential to avoid diagnostic delays and guide appropriate imaging and timely surgical management.