Which condition is a water balance-related cause of hyponatremia, including SIADH?

• A. SIADH
• B. Polydipsia
• C. Pseudohyponatremia
• D. In vitro hemolysis

Answer: (A)

Hyponatremia caused by too much free water relative to solutes is a water-balance problem. The classic example is SIADH, where inappropriate ADH secretion makes the kidneys reabsorb water even though sodium is low. This dilutes the serum sodium, giving hyponatremia with low plasma osmolality, and the urine remains inappropriately concentrated with high urine sodium, despite euvolemia and normal kidney, adrenal, and thyroid function. That pattern—dilutional hyponatremia driven by excess water retention—captures the essence of a water-balance–related cause.

Polydipsia also involves excess water, but in that scenario ADH would be suppressed as serum osmolality falls, leading to dilute urine rather than the concentrated urine seen in SIADH. Pseudohyponatremia is a lab artifact from abnormal serum constituents and does not reflect true water balance. In vitro hemolysis can artifactually alter some measurements but does not represent genuine hyponatremia from water balance. Thus SIADH best fits the concept of a water-balance–related cause of hyponatremia.