A Rare Case of Combined Syndrome of Inappropriate Antidiuretic Hormone Secretion and Fanconi Syndrome in an Elderly Woman

An 83-year-old Japanese woman was admitted to our hospital with severe hyponatremia (sodium, 108 mEq/L [108 mmol/L]), hypokalemia, hypochloridemia, hypocalcemia, hypophosphatemia, and hypouricemia. Despite low plasma osmolarity (232 mOsm/kg [232 mmol/kg]), urine osmolarity (320 mOsm/kg) was greater than that of plasma, and she had increased urinary sodium excretion (88 mEq/L [88 mmol/L]) and an unsuppressed high plasma level of antidiuretic hormone (ADH; 5.5 pg/mL [5.1 pmol/L]). These findings indicated that she had syndrome of inappropriate ADH secretion (SIADH). In addition, she had a generalized reabsorption defect of the proximal tubules, including decreased tubular reabsorption of phosphate, increased fractional excretion of uric acid, glycosuria despite a normal blood glucose level, and panaminoaciduria. Thus, combined SIADH and Fanconi syndrome was diagnosed. The cause was thought to be the antidepressant paroxetine, which is a selective serotonin reuptake inhibitor (SSRI). Several of the abnormal values resolved within 1 week after discontinuation of this drug. Although the precise mechanism responsible was not elucidated, we report an extremely rare case of combined SIADH and Fanconi syndrome, probably caused by short-term SSRI therapy.