Which statement about euvolemic hyponatremia is true?

Which statement about euvolemic hyponatremia is true?

• A. It is associated with hypervolemia
• B. It has normal extracellular fluid volume
• C. It is caused by dehydration
• D. It never involves SIADH

Answer: (B)

In euvolemic hyponatremia, the extracellular fluid volume is normal. The situation is dilutional: there’s excess free water that lowers serum sodium, but the overall sodium content hasn’t increased or decreased enough to create edema or dehydration, so the effective circulating volume stays normal. A common cause is SIADH, where inappropriate ADH release leads to water retention without expanding the extracellular fluid enough to cause edema. That’s why the statement about a normal extracellular fluid volume is true. The other ideas don’t fit: true hypervolemic hyponatremia involves edema and increased total body water (as in heart failure or cirrhosis); dehydration causes hypovolemic hyponatremia with reduced extracellular volume; and SIADH can be involved in euvolemic hyponatremia, not excluded by it.

In euvolemic hyponatremia, the extracellular fluid volume is normal. The situation is dilutional: there’s excess free water that lowers serum sodium, but the overall sodium content hasn’t increased or decreased enough to create edema or dehydration, so the effective circulating volume stays normal. A common cause is SIADH, where inappropriate ADH release leads to water retention without expanding the extracellular fluid enough to cause edema. That’s why the statement about a normal extracellular fluid volume is true. The other ideas don’t fit: true hypervolemic hyponatremia involves edema and increased total body water (as in heart failure or cirrhosis); dehydration causes hypovolemic hyponatremia with reduced extracellular volume; and SIADH can be involved in euvolemic hyponatremia, not excluded by it.