We studied thrombopoietic activity in hyperthyroidism by determination of reticulated platelet counts. At the time of hyperthyroidism 14/15 patients had higher reticulated platelets than after achievement of euthyroidism (P<0.001). There was no difference in peripheral platelet counts and mean platelet volumes at the time of hyperthyroidism when compared to euthyroidism. Three patients had pan- and auto-reactive platelet antibodies during hyperthyroidism. These antibodies were directed against GPIIb/IIIa in two patients and against GPIb/IX in one patient. Our findings provide direct evidence that hyperthyroidism is associated with increased platelet production, as reflected by an increase in reticulated platelets.