[Paroxetine-induced hyponatremia in an elderly man due to the syndrome of inappropriate secretion of antidiuretic hormone].

We report an 82-year old man prescribed paroxetine who had hyponatremia and in whom the syndrome of inappropriate secretion of antidiuretic hormone was diagnosed. He had taken sulpiride for depressed mental status. However, he showed parkinsonism, which was an adverse effect from the treatment of sulpiride. Therefore sulpiride was changed to selective serotonin reuptake inhibitor, paroxetine 10mg daily. His depressed mental status deteriorated after paroxetine treatment started. His depression had not lessened after 12 days, and the dosage was increased to 20mg daily. On the 15th day after starting paroxetine, routine laboratory tests showed that his serum sodium level was 126 mEq/l. We recognized that his confusion and loss of appetite were symptoms of hyponatremia, rather than of worsening depression. Laboratory data revealed hyponatremia, low serum osmolarity (242 mOsm/kg) with a relatively high level of serum antidiuretic hormone, and concentrated urine (439 mOsm/kg). We diagnosed the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), associated with paroxetine. The dosage of paroxetine was reduced gradually and the serum sodium level returned to normal on day 2 after medication ceased completely. Paroxetine produces fewer adverse effects than other types of antidepressants. However, its use can be associated with inappropriate secretion of antidiuretic hormone in the body and may lead to SIADH, which is characterized by hyponatremia, a potentially fatal condition that is typically asymptomatic until it becomes severe. SIADH is more likely in some populations, including the elderly. Serum sodium levels should be monitored closely, especially in elderly patients.