These investigators argued that potassium depletion predisposes the patients to hyponatremia because the serum sodium concentration is dependent upon the ratio of the sum of exchangeable sodium and potassium to total body water. They also speculated that potassium depletion might cause a shift of sodium into the intracellular space, 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