An uncommon cause of hypoosmolar hyponatremia is which condition?

• A. Adrenal insufficiency
• B. SIADH
• C. Primary polydipsia
• D. Hypothyroidism

Answer: (A)

Adrenal insufficiency is an uncommon cause of hypoosmolar hyponatremia. Hyponatremia with low serum osmolality arises when there is excess free water relative to solute, and failure to excrete free water is often driven by elevated ADH. Cortisol normally helps restrain ADH release; when cortisol is deficient, ADH rises inappropriately, promoting water retention and dilution of serum sodium. In primary adrenal failure, aldosterone deficiency adds salt wasting and potential hypovolemia, which further stimulates ADH and worsens hyponatremia. Because SIADH and primary polydipsia more typically account for hypoosmolar hyponatremia, adrenal insufficiency is considered a less common cause. If suspected, evaluate cortisol (or perform a cosyntropin test) and treat the underlying adrenal deficiency with glucocorticoids, which often improves the hyponatremia.