| I |
Nondiagnostic or unsatisfactory |
| Normal thyroid tissue only |
| Extrathyroid tissue only (e.g., skeletal muscle, mature adipose tissue) |
| A virtually acellular specimen |
| Acellular/paucicellular fibrotic nodule |
| Blood clot only |
| Other |
| II |
Benign lesion |
| Benign follicular nodule or consistent with a benign follicular nodule |
| Hashimoto's thyroiditis |
| Granulomatous (subacute) thyroiditis |
| Nonthyroidal lesion (e.g., parathyroid lesions, benign neurogenic tumors, benign lymph node) |
| Other |
| III |
Indeterminate lesion |
| IIIA |
Indeterminate follicular lesion with nuclear atypia |
| Follicular proliferative lesions with focal nuclear atypia |
| Follicular proliferative lesions with equivocal or questionable nuclear atypia |
| Atypical follicular cells embedded in a fibrotic stroma |
| IIIB |
Indeterminate follicular lesion with architectural atypia |
| Microfollicular proliferative lesion lacking a fibrous capsule or the adjacent nonlesional tissue in the specimen |
| Solid or trabecular follicular lesion lacking a fibrous capsule or the adjacent nonlesional tissue in the specimen |
| Macrofollicular proliferative lesion with a fibrous capsule |
| Hürthle cell proliferative lesion lacking a fibrous capsule or the adjacent nonlesional tissue in the specimen |
| IIIC |
Other indeterminate lesions |
| IV |
Follicular neoplasm or suspicious for a follicular neoplasm |
| IVA |
Follicular neoplasm with nuclear atypia |
| IVB |
Follicular neoplasm with architectural atypia |
| Microfollicular proliferative lesion with a fibrous capsule |
| Mixed microfollicular and normofollicular proliferative lesion with a fibrous capsule |
| Solid/trabecular follicular proliferative lesion with a fibrous capsule |
| IVC |
Follicular neoplasm, Hürthle cell type |
| Hürthle cell proliferative lesion with a fibrous capsule |
| V |
Suspicious for malignancy |
| Suspicious for papillary carcinoma, medullary carcinoma, poorly differentiated carcinoma, metastatic carcinoma, lymphoma, etc. |
| VI |
Malignant |
| Papillary thyroid carcinoma, poorly differentiated carcinoma, undifferentiated (anaplastic carcinoma), medullary thyroid carcinoma, lymphoma, metastatic carcinoma, etc. |
