Distant metastases as initial presentation of follicular carcinoma of the thyroid is rare, especially in young patients. We report the clinical and pathological features of a 33-year old pregnant patient with follicular carcinoma of the thyroid who presented with widespread bone and lung metastases at the time of diagnosis. the resected tumor had a focal insular component that showed extensive vascular invasion spreading beyond the thyroid capsule, and was associated with widespread bone and lung metastases. Despite its aggressive behavior, the tumor had low mitotic activity and Ki-67 nuclear labeling index. tumor cells showed high microvascular density and down-regulation of E-cadherin, a calcium-dependent trans-membrane epithelial protein molecule known to promote intercellular adhesion. We suggest that architectural differentiation of the tumor and cell proliferation rate are not reliable markers of metastatic behavior in this particular thyroid neoplasm. Microvascular density and down-regulation of E-cadherin expression in the tumor should be included among histologic hallmarks of metastatic potential. the role of pregnancy in the aggressive behavior of this tumor is discussed along with a literature review.

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