A study was carried out in which the platelet count was decreased in approximately half the patients with hyperthyroidism and gradually increased with treatment. Platelet disappearance curves were curvilinear and the platelet survival was shortened in the hyperthyroid state. Patients maintained in a euthyroid state for 3 months or less continued to have a shortened platelet survival. The survival returned to normal after 6 months or more of euthyroid status. In order to clarify the cause of the decreased platelet count in the patients, animal experiments were performed. T3-injected rats had decreased platelet counts and shortened platelet survival. When platelets obtained from T3-injected rats were transfused to a control group of untreated rats, the platelet survivals were normal. When platelets obtained from the control group of rats were transfused to T3-injected rats, the platelet survivals were shortened. Disappearance of heat-damaged RBC from the circulation was also accelerated in T3-injected rats. This suggests that thrombocytopenia in Graves’ disease is caused by an increased sequestration potency of the reticulo-endothelial phagocyte system stimulated by thyroid hormone.