Hyponatremia with increased plasma antidiuretic hormone in a case of hypothyroidism

We report a 70-year-old woman with hypothyroidism and severe hyponatremia, Her plasma antidiuretic hormone (ADH) level was inappropriately high for he r low plasma osmolality, Her low serum sodium level was gradually corrected by water restriction and sodium supplementation prior to the initiation of thyroid hormone replacement. After a diagnosis of Hashimoto's thyroiditis had been made, the patient was treated with levothyroxine. Following this t reatment, the patient's serum sodium level increased drastically, It is sug gested that the elevated plasma ADH level played an important role in the d evelopment of hyponatremia in this case.