Follicular-Patterned Lesions of the Thyroid Gland.

Follicular-patterned thyroid lesions are common and are composed of a wide spectrum of thyroid lesions, including nonneoplastic, benign/low-risk neoplastic, to malignant lesions. The pathologic classifications of these lesions are based predominantly on 3 features: (1) the invasion status, (2) the nuclear features of papillary thyroid carcinoma (PTC), and (3) the presence of >75% oncocytic cells. This review summarizes the diagnostic criteria, subtyping, pathologic features, differential diagnosis, clinical characteristics, and molecular findings of follicular-patterned thyroid lesions, including follicular nodular disease, follicular adenoma, follicular carcinoma, oncocytic adenoma, oncocytic carcinoma, noninvasive follicular thyroid neoplasm with papillary-like nuclear features, thyroid tumor of uncertain malignant potential, PTC infiltrative follicular subtype, and invasive encapsulated follicular variant of PTC.