Increased vasopressin excretion in patients with hypothyroidism.

Twenty-four h urinary vasopressin excretion was measured by bioassay in 15 patients with untreated hypothyroidism and compared with plasma sodium concentration. Four patients had raised excretion of an antidiuretic substance and in 3 of these patients excretion was reduced after thyroid replacement therapy. The criteria applied supported the view that the antidiuretic substance was arginine vasopressin. Plasma sodium concentration was normal in all these 4 patients. A further 4 patients had hyponatraemia without raised arginine vasopressin excretion. The results suggest that: (1) excess arginine vasopressin secretion is not the cause of the hyponatraemia of hypothyroidism and (2) an increased secretion of arginine vasopressin does occur in some cases of normonatraemic hypothyroidism, the cause requiring further elucidation.