WHO Histological Classification of Thyroid Tumors

I. Primary epithelial tumors

A. Follicular epithelial tumors

Benign: follicular adenoma.
Cross-sectional: follicular tumors of undetermined malignant potential, highly differentiated tumors of undetermined malignant potential, noninvasive follicular tumors with papillary nuclear features, and hyaline metaplastic beam-like tumors.
Malignant: thyroid cancer, including ① Differentiated thyroid cancer: PTC,
FTC, eosinophilic carcinoma; ②PDTC; ③ATC.

B. MTC

C. Mixed follicular epithelial and parafollicular cell tumors

II. Primary non-epithelial tumor

A. Paraganglioma and mesenchymal tumors

B. Tumors of the lymphohematopoietic system

C. Germ cell tumors

D. Other

III. Secondary tumors