- The most common cause of euvolemic hyponatremia is Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH).

- In SIADH, there is an excess release of antidiuretic hormone (ADH) despite normal or low plasma osmolality.
- This causes the kidneys to retain water, leading to dilution of sodium in the blood without causing significant changes in total body sodium or extracellular fluid volume.
- Therefore, patients typically present with low serum sodium levels but normal extracellular fluid volume (euvolemia).

- In contrast, Congestive Heart Failure (CHF) usually causes hypervolemic hyponatremia due to fluid retention and edema.
- Similarly, Nephrotic Syndrome leads to hypoalbuminemia and edema with hypervolemic hyponatremia. Addison's disease may cause hyponatremia, but it is generally associated with hypovolemia due to aldosterone deficiency.

Hence, among the options listed, SIADH is the classic and most frequent cause of euvolemic hyponatremia.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Chapter 364, Page 2736