thereby further compromising the extracellular volume and stimulating vasopressin release. sodium and/or potassium depletion alone would not be expected to cause marked hyponatremia because water excretion normally increases as the serum sodium concentration falls. However, because thiazides interfere with renal water excretion, they impair this normal osmoregulatory response. Thus, as in most patients with hyponatremia, urine osmolality is inappropriately high relative to the osmolality of plasma and often exceeds it