Syndrome of inappropriate antidiuresis in Waldenström's macroglobulinemia.

Hyponatremia due to the syndrome of inappropriate antidiuresis rather than due to isotonic hyponatremia from hyperproteinemia developed in a patient with Waldenström's macroglobulinemia. The patient was unable to excrete a water load normally despite suppression of antidiuretic hormone to normal levels. The temporal relationship between control of the tumor and resolution of the hyponatremia suggests that the tumor either produced a substance that enhanced the hydro-osmotic effect of endogenous antidiuretic hormone or produced an antidiuretic substance immunologically different from antidiuretic hormone. The syndrome of inappropriate antidiuresis should be suspected in hyponatremic patients with Waldenström's macroglobulinemia.