Stage IV includes: IVA: 55 years and older: T4b, Any N, M0; IVB: 55 years and older: Any T, Any N, M1. T4b: Gross extrathyroidal extension invading prevertebral fascia or encasing the carotid artery or mediastinal vessels from a tumor of any size. M0: No distant metastasis. M1: Distant metastasis. (from AJCC 8th Ed.) [from NCI]