[Hyponatraemia].

Because hyponatraemia can be caused by many disorders, the diagnostic approach to hyponatraemia can be challenging for physicians. Causes of hyponatraemia can be classified according to a combination of laboratory parameters (e.g. sodium levels and osmolality in serum and urine) and clinical parameters (e.g. volume status, medication). Based on the description of two patient cases, the differential diagnosis of hyponatraemia is discussed by combining these parameters.