Because of an increased incidence of thromboembolic disease associated with thyroiditis, thyroid hormone has been suspected of impacting the normal thrombohemorrhagic balance. However, the effect of the hormone has never been studied in the absence of inflammatory thyroid disease. We have isolated the effect of thyroid hormone by studying it in patients with a history of thyroid cancer who had had total thyroidectomies. Blood samples were obtained from these patients when they were taking suppressive doses of thyroxin and were therefore chemically hyperthyroid and again after they had discontinued their replacement in order to have a I131 thyroid scan and were chemically very hypothyroid. A series of analytes related to coagulability and fibrinolysis were measured. We found that when the patients were hyperthyroid they had elevated levels of prothrombin fragment F1+2, factor VIII, and plasminogen activator inhibitor-1, all of which suggest that thyroid hormone in prothrombotic.