The serum sodium concentration and thus serum osmolality are closely controlled by water homeostasis, which is mediated by thirst, arginine vasopressin, and the kidneys.¹ A disruption in the water balance is manifested as an abnormality in the serum sodium concentration — hypernatremia or hyponatremia.²,³ Hypernatremia, defined as a rise in the serum sodium concentration to a value exceeding 145 mmol per liter, is a common electrolyte disorder. Because sodium is a functionally impermeable solute, it contributes to tonicity and induces the movement of water across cell membranes.⁴ Therefore, hypernatremia invariably denotes hypertonic hyperosmolality and always causes cellular dehydration, at . . .