We evaluated the causal role of glucocorticoid deficiency in the hyponatremia that developed in a 57-year-old Japanese man with hypothyroidism following the performance of a total thyroidectomy for laryngeal cancer. The plasma concentration of vasopressin (1.78 pg/ml) was not suppressed in the presence of hyponatremia (125 mEq/l). The urinary excretion of sodium was increased, and the plasma renin activity and plasma aldosterone concentration were suppressed. The infusion of hypertonic saline increased the plasma osmolality, but not the plasma concentration of vasopressin. An oral water load (20 ml/kg of body weight) did not suppress the plasma vasopressin level or induce diuresis. Pretreatment with hydrocortisone normalized the response of plasma vasopressin to the water load was well as the diuretic response during the hypothyroid state. The urinary excretion of 17-hydroxycorticosteroids was below normal in the hypothyroid state in the face of normal serum cortisol concentration. The correction of the hypothyroidism returned these abnormalities to normal. A disturbed metabolism of glucocorticoid may have been responsible for the hyponatremia and disturbance in plasma vasopressin regulation observed in this hypothyroid patient.