Wereport a 70-year-old womanwith hypothyroidism and severe hyponatremia. Her plasma antidiuretic hormone (ADH) level was inappropriately high for her 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 treatment, the patient's serum sodium level increased drastically. It is suggested that the elevated plasma ADHlevel played an important role in the development of hyponatremia in this case.