Because of menorrhagia, a 13-year-old girl was found to have type I von Willebrand disease and then chronic autoimmune thyroiditis with hypothyroidism. All clinical and laboratory evidence of von Willebrand disease resolved transiently after infusion of desmopressin, and permanently with L-thyroxine therapy. We recommend investigation for hypothyroidism in patients with newly diagnosed acquired von Willebrand disease.