What is one of the most common causes of hyponatremia?

• A. Hyperkalemia
• B. Hypovolemia
• C. SIAD
• D. Diabetes insipidus

Answer: (C)

Excess free water relative to sodium drives hyponatremia, and one of the most common causes is Syndrome of Inappropriate Antidiuretic Hormone Secretion. In SIAD, antidiuretic hormone (ADH) is released inappropriately, so the kidneys reabsorb more water even when serum osmolality is low. That extra water dilutes the serum sodium, leading to dilutional hyponatremia. Clinically this often presents as euvolemic hyponatremia, with a low serum osmolality and inappropriately concentrated urine (high urine osmolality) because ADH is still acting.

Hyperkalemia is not the driver of hyponatremia. Diabetes insipidus would cause increased water loss and typically hypernatremia rather than hyponatremia. Hypovolemia can cause hyponatremia in some situations, but SIAD is a more frequent cause across many patient populations, especially in hospitalized individuals and those with certain medications or illnesses.