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Background: Hyperthyroid cats might have a predisposition to arterial thrombus formation. The mechanism for thrombogenesis currently is unknown but could be associated with systemic hypercoagulability as seen in hyperthyroid humans. Objective: Our purpose was to evaluate markers of hemostasis in hyperthyroid cats compared to healthy cats, and in hyperthyroid cats before and after radioactive iodine treatment (RIT). Animals Twenty‐five cats with hyperthyroidism and 13 healthy euthyroid cats >8 years of age. Methods: Prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen concentration, antithrombin (AT), D‐dimers, thrombin‐antithrombin complexes (TAT), von Willebrand Factor antigen (vWF : Ag), and activity of factors VIII and IX were measured. An echocardiogram was performed in all cats. Hemostatic markers and echocardiogram were evaluated again 6 to 9 months after successful RIT in 7 cats. Results: Hyperthyroid cats had higher fibrinogen concentration (P <.0001), AT activity (P <.0001), and vWF : Ag concentration (P =.01) than healthy control cats with all results decreasing significantly post‐RIT. Hyperthyroid cats were not more likely to be in a hypercoaguable state than euthyroid cats (P =.08). Serum T4 concentration was not a predictor of a hypercoagulable state (P =.53). Conclusions and Clinical Importance: Hyperthyroid cats have evidence of altered hemostasis that does not appear to be solely attributable to cardiac abnormalities, but no evidence of a hypercoagulable state. Findings suggest altered hemostasis resolves after RIT. Hyperthyroid cats could have endothelial dysfunction as indicated by increased vWF : Ag which could potentiate thrombogenesis. [ABSTRACT FROM AUTHOR]