An investigation into the cause of hyponatremia in the syndrome of inappropriate secretion of antidiuretic hormone.

Abstract A clinical study using balance technics and measurements of the volumes of extracellular and intracellular fluid have shown that severe hyponatremia may be induced in the syndrome of inappropriate antidiuretic hormone secretion without unusual losses of sodium from the body or dilution of the plasma sodium. Evidence has been presented in support of the view that redistribution of sodium within the body may occur and the tentative conclusion was reached that sodium accumulated within the cells. The suggested sequence of events when patients with this syndrome are gradually water-loaded at a time when the serum sodium is normal is the following: Expansion of total body water → intracellular hypotonicity → shift of sodium from extracellular to intracellular fluid → relative rise in intracellular osmolality → shift of water into the cells.