A case of SIADH induced by mizoribin administration.

We describe a 74-year-old man with rheumatoid arthritis (RA) who developed syndrome of inappropriate secretion of antidiuretic hormone (SIADH) 1.5 months after commencement of mizoribin prescription when his arthritis was improved. He noticed nausea and headache and serum Na fell as low as 118 mEq/l. Normal urinary Na excretion without hypotension or hemoconcentration negated the possibility of dehydration resulting from urinary Na loss. Serum antidiuretic hormone (ADH) remained elevated at 0.59 pg/ml in spite of a significant reduction in serum osmolality to 254 mosm/kg. He had no organic disease likely to cause SIADH. Despite infusion of hypertonic saline, his serum Na was not restored to normal. Shortly after mizoribin withdrawal, his serum Na increased significantly from 128 to 139 mEq/l and plasma osmolality from 265 to 287 mosm/kg. ADH hypersecretion in relation to plasma osmolality was reversed by mizoribin withdrawal, suggesting that bredinin might adversely induce SIADH. Additional predisposing factors were the patient's age and difficulty in urination due to benign prostatic hypertrophy. In summary, we report herein the first case of SIADH believed to be an adverse effect of mizoribin, which may therefore needed to be added to the list of drugs which can induce SIADH.