Thyroid cancer staging (AJCC)

TX	Primary tumor cannot be assessed
T0	No evidence of primary tumor
T1	Tumor 2 cm / less in its greatest dimension, limited to thyroid
T1a	Tumor 1 cm or less limited to thyroid
T1b	Tumor more than 1cm but less than 2 cms and limited to thyroid
T2	Tumor size more than 2cms but not more than 4 cms and is limited to thyoid
T3	Tumor more than 4 cms limited to thyoid / tumor of any dimension with minimal extrathyroid extension (sternomastoid muscle, perithyroid soft tissues)
T4a	Moderately advanced disease. Tumor of any size extending beyond the confines of thyroid capsule to invade soft tissues, larynx, trachea, oesophagus or recurrent laryngeal nerve
T4b	Very advanced disease. Tumor invades prevertebral fascia / encases carotid sheath, mediastinal involvement
	All anaplastic carcinomas of thyroid are T4 tumors. T4a includes anaplastic carcinoma within thyroid tissue and T4b includes anaplastic carcinoma thyroid with extra thyroid involvement.

Surgery is indicated up to stage T4a.  Any stage above this level should be treated with palliative radioactive ablation.