A 71-year-old woman with altered consciousness was diagnosed with primary hyperparathyroidism (PHPT) after presenting with severe hypercalcemia (serum Ca 15.3 mg/dL) and parathyroid adenoma. Unusually, her 1,25(OH)2D3 level was undetectable, despite PHPT. Hemodialysis and evocalcet failed to control the calcium levels, but denosumab effectively reduced hypercalcemia. However, both 25(OH)D3 and 1,25(OH)2D3 remained low. This case highlights the importance of vitamin D assessment in PHPT, as its metabolism can vary. Denosumab may be a viable treatment option for calcimimetic hypercalcemia.