This study was undertaken to investigate the effects of hypothyroidism on buccal mucosal bleeding time and von Willebrand factor antigen (vWf:Ag) concentrations. Hypothyroidism was induced in 8 adult dogs by administration of iodine 131. Four healthy dogs acted as controls. Measurement of plasma vWf:Ag and serum thyroxine and triiodothy-ronine concentrations, and buccal mucosal bleeding time were made before induction of hypothyroidism, for 23 weeks after 131I administration, and during 5 weeks of levothyroxine supplementation. No significant changes in buccal mucosal bleeding times were noted during the study. After an insignificant increase in vWfAg concentration in hypothyroid dogs, levothyroxine treatment was associated with a significant decrease in vWf:Ag concentration in hypothyroid dogs when compared with controls. Results of this study suggest that hypothyroidism does not induce acquired von Willebrand's disease or significant defects in primary hemo-stasis. J Vet Intern Med 1996;10:60–64. Copyright © 1996 by the American College of Veterinary Internal Medicine.