Background: Extramedullary hematopoiesis (EMH) is the production of elements of erythroid and myeloid series at ectopic sites; when concomitant with agnogenic myeloid metaplasia, it is invariably seen in advanced disease. In EMH, involvement of the thyroid gland is extremely rare.
Case: An 82-year-old male with thyroid enlargement underwent fine needle aspiration (FNA) cytology with a diagnosis of malignancy. A month later another FNA cytology was performed and was consistent with EMH. A complete hematologic workup subsequently allowed the diagnosis of agnogenic myeloid metaplasia.
Conclusion: The presence of EMH in the thyroid gland is an unusual finding, and, due to the presence of numerous giant cells, the cytologic presentation might be mistaken for anaplastic thyroid carcinoma. The differential diagnosis is based on the recognition of giant cells as megakaryocytes.