Extramedullary hematopoiesis involving the thyroid: Fine-needle aspiration features and literature review

The thyroid has rarely been documented as a site of extramedullary hematopoiesis (EMH). We report the largest series to date, with nine cases of EMH, and compare our findings with previous reports of thyroid EMH.

Thyroid nodule fine-needle aspirations (FNAs) were collected over a 4-year period. Thin layer preparations were examined and correlated with clinical features and ultrasound characteristics. A comprehensive review of the English literature was done, and the results were compared with the current series.

During the study period, 172,939 thyroid FNAs were examined. Nine samples (0.005%) contained trilineage bone marrow elements. Nodule calcifications were present in 7 patients. None of the patients had a history of a blood disorder, nor was there any evidence of a thyroid malignancy. Fifteen reports of 18 patients with thyroid EMH were identified in the English literature. Nodule calcifications were reported in 10 patients. Thyroid EMH was associated with primary myelofibrosis in 4 patients, and with chronic anemia in 1; calcifications were absent in 3 patients, and were not specified in the remaining 2. None had evidence of a thyroid malignancy.

Thyroidal EMH is an extremely uncommon finding. Clinical and sonographic features are nonspecific. Thyroid EMH is usually an incidental finding, most likely related to mature osseous metaplasia. An occasional association with hematologic disease has been reported, however. To date, there does not appear to be any association with thyroid malignancy and surgery is typically not indicated. Therefore, recognition at the time of FNA is essential.