Chronic idiopathic hyponatremia in an elderly patient due to inappropriate antidiuretic hormone secretion (SIADH) syndrome.

INTRODUCTION: The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is a disorder which is characterized by the inability to suppress the secretion of antidiuretic hormone (ADH), leading to impaired water excretion and hyponatremia. The syndrome should be suspected in any patient with hyponatremia, hypo-osmolality and a urine osmolality >100 mOsm/kg, while urine sodium concentration is above 40 mEq/L. CASE DESCRIPTION: Herein, we present an 84-year-old female patient with chronic idiopathic hyponatremia due to SIADH. Her laboratory tests showed hyponatremia with serum sodium of 120 mEq/L, while urine sodium concentration was 83 mEq/L. Measured serum osmolality was 255 mOsm/kg and urinary osmolality 130 mOsm/kg. In addition to these, her serum glucose, potassium, uric acid, renal, and liver functions were normal, and there were no acid-base disorders. The patient's adrenal function (cortisol, adrenocorticotropic hormone, renin, and aldosterone) showed no abnormalities, as well as her thyroid function. DISCUSSION: The patient suffered from chronic idiopathic hyponatremia and osteoporosis, which often coexists in patients with chronic idiopathic SIADH and was treated with alendronate/cholecalciferol. The scenario of the presence of SIADH was further strengthened by the fact that hyponatremia did not improve after isotonic normal saline administration, but only with fluid restriction.  HIPPOKRATIA 2019, 23(1): 42-44. Copyright 2019, Hippokratio General Hospital of Thessaloniki.