What is the most common cause of euvolemic hyponatremia?

The most common cause of euvolemic hyponatremia is the syndrome of inappropriate antidiuretic hormone secretion (SIADH). In SIADH, excess antidiuretic hormone (ADH) leads to water retention, diluting sodium levels in the blood without significant changes in overall fluid volume. Other causes can include medications, adrenal insufficiency, and certain malignancies, but SIADH remains the primary etiology.