Severe Hypercalcemia after Vitamin D Supplementation in Primary Hyperparathyroidism: A Case Report

Hypercalcemia is an electrolyte imbalance with multiple etiologies, the most common being primary hyperparathyroidism (PHPT). The diagnostic workup of PHPT is relatively straightforward, as elevated serum calcium levels in the presence of elevated or inappropriately normal parathyroid hormone (PTH) immediately raise the suspicion for PHPT. However, this typical presentation can be unhinged by the presence of severe vitamin D deficiency, which may mask hypercalcemia. Furthermore, vitamin D deficiency itself can contribute to PTH increase.

We report the case of a 53-year-old man presenting with markedly elevated PTH levels (639 pg/ml) with only borderline-elevated serum calcium (10.8 mg/dl) and severe vitamin D deficiency (5 ng/ml). Once the vitamin D level was restored to normal range with cholecalciferol supplementation, the patient developed severe hypercalcemia (14.4 mg/dl) requiring specific management.

This case report offers an interesting insight into the potential for cholecalciferol replacement therapy to unmask significant hypercalcemia in PHPT patients and underscores the need for caution in such a context.