Differentiated thyroid cancer staging refers to TNM staging of papillary thyroid cancer and follicular thyroid cancer. Anaplastic thyroid carcinoma and medullary thyroid carcinoma are staged separately. The following article reflects the 8^th version published by the American Joint Committee on Cancer, which is used for staging starting January 1, 2018 ^1,2.

Primary tumour (T)

• TX: primary tumour cannot be assessed 

• T0: no evidence of primary tumour

• T1: tumour ≤2 cm in greatest dimension limited to the thyroid

  • T1a: tumour ≤1 cm, limited to the thyroid

  • T1b: tumour >1 cm but ≤2 cm, limited to the thyroid

• T2: tumour >2 cm but ≤4 cm, limited to the thyroid

• T3: tumour >4 cm limited to the thyroid or gross extrathyroidal extension invading only strap muscles

  • T3a: tumour >4 cm, limited to the thyroid

  • T3b: gross extrathyroidal extension invading only strap muscles (sternohyoid, sternothyroid, thyrohyoid, or omohyoid muscles)

• T4: gross extrathyroidal extension beyond the strap muscles

  • T4a: tumour invading subcutaneous soft tissues, larynx, trachea, oesophagus, or recurrent laryngeal nerve

  • T4b: tumour invading prevertebral fascia or encasing carotid artery or mediastinal vessels

Regional lymph node (N)

• NX: regional lymph nodes cannot be assessed

• N0: no regional lymph node metastasis

  • N0a: one or more lymph nodes confirmed benign by cytology/histology

  • N0b: no radiologic or clinical evidence of regional lymph node metastasis

• N1: regional lymph node metastasis

  • N1a: central lymph node metastases to level VI (pretracheal, paratracheal, and prelaryngeal/Delphian lymph nodes) or level VII (upper mediastinal); can be unilateral or bilateral

  • N1b: lateral neck lymph node metastases (levels I, II, III, IV, or V, or retropharyngeal); can be unilateral, bilateral, or contralateral

Distant metastases (M)

The terms pM0 and MX are not valid TNM categories. The following categories may be used:

• cM0: no evidence of metastases

• cM1: distant metastasis

• pM1: distant metastasis, microscopically confirmed

Stage groups

If age at diagnosis <55 years:

• stage I

  • [Any T], [Any N], M0

• stage II

  • [Any T], [Any N], M1

If age at diagnosis ≥55 years:

• stage I

  • [T1, T2], [N0, NX], M0

• stage II

  • [T1, T2], N1, M0

  • T3, [Any N], M0

• stage III

  • T4a, [Any N], M0

• stage IVA

  • T4b, [Any N], M0

• stage IVB

  • [Any T], [Any N], M1

Changes since prior versions

Compared to the 7^th edition, stage group criteria were changed, including raising the age cutoff from 45 to 55 years. Minor extrathyroidal extension (detected histologically only) and lymph node metastases were removed as T3 and stage III criteria, respectively. Anaplastic tumours are no longer automatically classified as T4 disease and stage groups for anaplastic thyroid cancer have been modified to use the T category definitions used for differentiated thyroid cancer.