Hematologic manifestations of thyroid disease

The endocrine system plays an important role in the regulation of normal hematopoiesis and, to a certain extent, the coagulation system. The erythrocytes appear to bear the brunt of thyroid dysfunction, with anemia being the most common finding in hypothyroidism and, to a certain extent, hyperthyroidism, even though erythroid hyperplasia can occur in the latter. Thyroid hormone effects on leukocytes appear to be minimal, but myeloid hyperplasia does occur in thyrotoxicosis. In addition, an increase in the incidence of lymphoproliferative and myeloproliferative disorders has been reported in these patients, In the case of the hemostatic system, thrombocytopenia in the setting of thyrotoxicosis is the most striking finding, with severe decreases in platelet counts associated with immune-mediated mechanisms. The activity levels of the various coagulation factors can be diminished in patients with hypothyroidism, probably as a result of the global decrease in protein synthesis seen in this disorder. However, these abnormalities of the clotting factors do not appear to result in significant bleeding or clotting problems. Awareness of the hematologic abnormalities associated with thyroid dysfunction should enhance the understanding of these often complex problems.