Which of the following is NOT a cause of hyponatremia?

• A. Diabetes insipidus
• B. Hyperglycemia
• C. Hyperaldosteronism
• D. Thiazide diuretic use

Answer: (A)

Hyponatremia arises when there is more free water relative to sodium in the body or when sodium is lost and not adequately replaced. Diabetes insipidus does the opposite: it causes the kidneys to excrete large amounts of free water, leading to hypernatremia rather than hyponatremia. So it is not a cause of low serum sodium.

Hyperglycemia raises serum osmolality, pulling water out of cells and diluting the serum sodium, which can produce a true hyponatremia after accounting for the free-water shift. In contrast, hyperaldosteronism causes sodium retention and tends to raise, not lower, serum sodium, leading toward hypernatremia. Thiazide diuretics promote water retention relative to sodium excretion in the distal nephron, which can dilute serum sodium and trigger hyponatremia.

Thus, the statement describing diabetes insipidus as causing hyponatremia is not accurate, making it the correct choice for which option is NOT a cause of hyponatremia.