There is a growing epidemic of thyroid nodules, commonly detected as incidental imaging findings. The vast majority of nodules are benign and of primary thyroidal origin. However, a multitude of non-native, extrinsic or systemic conditions may affect the thyroid and mimic primary thyroid nodules. Contributing factors may include the glands' location in an anatomically dense area, rich vascular and lymphatic network, and embryological origin. In this article we describe a variety of extrinsic and unusual pathology which can affect the thyroid gland. Conditions are classified into benign congenital, benign acquired, cancers which secondarily involve the thyroid gland and unusual cancers arising from within the gland itself. The imaging findings, primarily on high-resolution ultrasound, are reviewed and illustrated with examples. Where possible, imaging features which suggest a specific pathological category or entity are highlighted. It is important that those performing ultrasound examination of the thyroid gland are aware that thyroid nodules may not exclusively represent pathology native or intrinsic to the gland itself.
Keywords: Metastasis; Thyroid gland; Thyroid nodule; Ultrasound.
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