[Increased plasma antidiuretic hormone in the presence of hyponatremia in primary hypothyroidism].

A 70-year-old woman was admitted because of disturbance of her consciousness. Physical examinations and laboratory data suggested hypothyroidism. Primary hypothyroidism was subsequently confirmed with endocrinological examinations. Antidiuretic hormone (ADH) levels were elevated despite severe hyponatremia. On admission, urinary sodium concentration was 10mEq/l. The patient was treated with saline intravenously; serum sodium level increased from 120 to 125mEq/l and urinary sodium concentration increased from 10 to 54mEq/l. Mental confusion developed and serum sodium level dropped with urinary sodium concentration above 20mEq/l when thyroid replacement was started with the cessation of saline infusion. The patient's state of consciousness, elevated ADH levels, decreased serum sodium level and urinary sodium concentration were improved by thyroid replacement together with hydrocortisone therapy. Effects of acute water loading were abnormal with the administration of iodothyronine (T3) alone but were normalized with the administration of hydrocortisone together with T3. On discharge she was treated with the oral administration of levothyroxine alone. Pituitary hormones were normal. These results suggest that the patient was in a state of hypoadrenocorticism. Impaired water excretion in a state of hypoadrenocorticism due to hypothyroidism may give rise to an inappropriate secretion of ADH thereby resulting in hyponatremia, which in turn leads to hypotonic dehydration induced by water intoxication.