BACKGROUND Severe hypercalcemia is a life-threatening medical emergency. Its possible etiologies are hyperparathyroidism, malignancy, chronic granulomatous diseases, vitamin D intoxication, medication (lithium, thiazine, excessive vitamin A), hyperthyroidism, acromegaly, adrenal insufficiency, pheochromocytoma, milk-alkali syndrome, and immobilization. Dehydration is considered a consequence of hypercalcemia but not the etiology. CASE REPORT We present a case of a 50-year-old woman who presented with headache, nausea, dizziness, and profound weakness, with a serum calcium level of 17.3 mg/dL due to severe dehydration. The other causes of hypercalcemia were diligently excluded with extensive laboratory testing. The patient's calcium level improved with aggressive intravenous hydration. In the subsequent follow-up visits, the calcium level remained within the reference range. The pathophysiology of severe hypercalcemia caused by dehydration is not clear. However, a feedforward mechanism has been proposed to occur, which worsens both dehydration and hypercalcemia. Dehydration as an initial insult leads to mild or transient hypercalcemia due to decreased fluid volume that affects calcium excretion via the kidneys. Subsequently, hypercalcemia interferes with the kidney's ability to concentrate urine, leading to further dehydration. This sets up a vicious loop that worsens both dehydration and hypercalcemia, leading to profound dehydration and severe hypercalcemia. CONCLUSIONS Dehydration is considered a consequence of hypercalcemia but has not been identified as the etiology of severe hypercalcemia. Hyperparathyroidism and malignancy are the most common causes of severe symptomatic hypercalcemia, and dehydration is the diagnosis of exclusion. However, it is imperative to keep dehydration in the differential diagnosis for a patient presenting with severe symptomatic hypercalcemia, as highlighted by our case.