The staging of thyroid cancer follows the same internationally accepted staging system of T (for primary tumor lesions), N (for lymph node metastases), and M (for distant metastases).
Unlike other tumors that are classified as stage I-IV, undifferentiated thyroid cancer is classified as advanced (stage IV) at diagnosis because of its extreme malignancy. Depending on the extent of the primary focus and the presence or absence of metastases, it can be subdivided into stages IVA, IVB, and IVC:
| Staging | Meaning |
| IVA period | No lymph node metastases or distant metastases, primary focus confined to the thyroid |
| Stage IVB |
No lymph node metastasis and distant metastasis with significant invasion of the primary focus into extrathyroidal tissue; or no distant metastases, with lymph node metastases and primary foci confined to the thyroid gland |
| Stage IVC | Distant metastases, regardless of the extent of the primary focus or the presence of lymph node metastases |
Co-written by Dr. Ben Ma, Cancer Hospital of Fudan University