[Unusual non-neoplastic lesions in the "surgical pathology" of the thyroid]

This review aims to describe and assist in the categorization of most of the unusual non-neoplastic conditions, encountered in the surgical pathology of the thyroid. The conditions included are: normal intrathyroidal vestigial tissues/structures (i.e. rests of the ultimobranchial body and thyroglossal duct) and their relevant pathological derivatives (ultimobranchial body cyst, intrathyroidal lymphoepithelial cyst, thyroglossal duct cyst); mature intrathyroidal heterologous tissues/organs of either metaplastic or heterotopic origin (adipose tissue, striated skeletal muscle, cartilage, parathyroid glands, thymus, salivary gland tissue) and their relevant pseudotumoural lesions; varieties of metaplastic and non-metaplastic morphologic changes of the thyroid follicular epithelium (oncocytic, clear cell/signet ring cell, darkly pigmented cell, mucinous (myxoid), squamous, spindle cell); amyloid goiter; some reactive and/or degenerative cytologic and nuclear atypicalities (nuclear pseudoclearing and cell pleomorphism) as well as some hyperplastic or peculiar growth patterns (capsular pseudoinvasion; vascular invasion; papillary carcinoma-like and paraganglioma-like patterns) of benign conditions mimicking neoplasia; and finally. some pseudotumoural lesions of the stroma (pseudoangiosarcomatous vascular proliferation, and post-fine-needle aspiration spindle cell nodule). The pathogenetic mechanism, the morphologic interpretation, and the differential diagnosis of each of the above-listed conditions are discussed and pertinent illustrations for many of them are also provided. Lesions of thyroid tissue situated outside of the gland itself are not discussed.