Heterotopic bone formation with extramedullary haematopoiesis in a thyroid nodule.

The presence of mature bone and bone marrow in the thyroid gland is an exceedingly rare occurrence. Extramedullary haematopoiesis (EMH) and heterotopic bone formation (HBF) should be suspected when cytology of thyroid nodules reveals evidence of megakaryocytes or bone marrow fat, respectively. The cause of these abnormalities has not been fully elucidated, but the role of bone morphogenic factors (BMPs) in their pathogenesis has been suggested. Both EMH and HBF can be seen in both benign and malignant primary thyroid conditions, and although they have not been definitively associated with significant pathology, it is recommended that extension studies be considered in the event of these findings to rule out concomitant haematological conditions.