Follicular-Patterned Lesions of the Thyroid Gland

Follicular-patterned thyroid lesions are common and are composed of a wide spectrum of thyroid lesions, including non-neoplastic, benign/low-risk neoplastic, to malignant lesions. The pathologic classifications of these lesions are based predominantly on three 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 (NIFTP), thyroid tumor of uncertain malignant potential, papillary thyroid carcinoma infiltrative follicular subtype, and invasive encapsulated follicular variant of papillary thyroid carcinoma.