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Background/objective: Primary hyperparathyroidism is the most common cause of hypercalcemia. Hypercalcemia secondary to giant parathyroid adenomas and parathyroid carcinomas share substantial clinical similarities and their differentiation is challenging. Case report: We present the case of a 72-year-old female with profound lethargy and confusion associated with polydipsia and polyuria, who was found to have an extremely elevated serum calcium level of 18.3 mg/dL (normal 8.4–10.5 mg/dL), ionized calcium 2.56 mmol/L (normal 1.20–1.32 mmol/L) and intact parathyroid hormone of 1838 pg/mL (normal 15–65 pg/mL). Ultrasound revealed a large hypoechoic nodule measuring 2.5 x 1.8 × 2.0 cm located in the right thyroid bed. Computed tomography revealed a hypoattenuating mass in the same region without obvious soft tissue invasion or lymphadenopathy. Technetium-99 m sestamibi with single-photon emission computed tomography identified intense focal radiotracer uptake at the same location most consistent with hyperfunctional parathyroid tissue. Patient underwent surgical exploration and resection of a parathyroid mass, which weighed 12.1 g and measured 4.6 x 2.5 × 1.8 cm. Pathology confirmed a diagnosis of benign parathyroid adenoma. Patient's laboratory values normalized and her mental status improved significantly, and she was discharged in stable condition. Discussion: This is a rare case of a symptomatic patient with extreme primary hyperparathyroidism and hypercalcemia that was found to have a benign giant parathyroid adenoma, rather than carcinoma. Conclusion: Differentiation between large hyperfunctioning parathyroid adenoma and parathyroid carcinoma is nontrivial due to shared characteristics of each. Therefore, the final diagnosis requires imaging and histopathological examination.