Hyponatremia of hypothyroidism. Appropriate suppression of antidiuretic hormone levels.

A hypothyroid, 72-year-old woman with idiopathic hypopituitarism manifested severe hyponatremia, plasma hypoosmolality, and inappropriately elevated urine osmolality suggestive of a syndrome of inappropriate antidiuretic hormone secretions. The hyponatremia did not respond to demeclocycline hydrochloride, and antidiuretic hormone (ADH) levels measured by a specific radioimmunoassay were appropriately suppressed. Subsequent replacement therapy with levothyroxine sodium resulted in correction of the hyponatremia. Thus, both direct assay as well as hormone blockade failed to show an action of ADH in mediating the water retention.