Hyponatremia in decompensated heart failure is primarily due to which mechanism?

• A. Hyperkalemia due to aldosterone
• B. Hypernatremia due to dehydration
• C. Hyponatremia due to neurohormonal activation and water retention
• D. Hypocalcemia due to PTH

Answer: (C)

In decompensated heart failure, hyponatremia mainly comes from dilutional water retention driven by neurohormonal activation. When the heart’s pumping ability drops, the effective circulating volume falls, and the body kicks in with systems that conserve both sodium and water to preserve perfusion: the renin-angiotensin-aldosterone system, increased sympathetic activity, and especially antidiuretic hormone (vasopressin) release. Vasopressin acts on the kidneys to reabsorb free water in the collecting ducts. This water retention outpaces sodium retention, so the serum sodium concentration drops even though total body sodium and water may be high. The result is hyponatremia.

The other options don’t fit this mechanism: aldosterone-driven potassium effects don’t cause hyponatremia, dehydration would raise sodium (hypernatremia), and PTH-related calcium changes are unrelated to acute sodium balance.