Follicular cancers are classified into minimally invasive and (extensively) invasive categories. Patients with minimally invasive follicular cancer have an excellent prognosis. The presence of capsular invasion or, at most, a few areas of blood vessel invasion distinguishes minimally invasive follicular cancer from a benign cellular follicular adenoma. In contrast, patients with follicular cancer with extensive vascular invasion have a poorer prognosis; distant metastases in lung or bones are sometimes present at the time of diagnosis.

Almost all endocrinologists agree that therapy for patients with invasive follicular cancer should consist of a total or near-total thyroidectomy, usually followed by a radioactive iodine ablation of remnant thyroid tissue.[16] Lifelong thyrotropin suppression may be indicated for such patients, by using Tg and periodic radioiodine scans (where indicated) to monitor for recurrence. The optimal therapy for minimally invasive follicular cancer is more controversial. Some endocrinologists recommend postoperative radioactive iodine scanning and ablation therapy (when appropriate), followed by thyrotropin suppressive therapy. Minimally invasive follicular cancer is often indistinguishable from benign follicular neoplasms at the time of surgery, even by frozen section; a definitive diagnosis often requires multiple sections through the primary tumor. When a lobectomy alone has been performed and minimally invasive follicular cancer is diagnosed after a histologic examination has been completed, 3 alternative therapies may be considered: (1) levothyroxine suppressive therapy alone, (2) completion thyroidectomy followed by radioactive iodine scanning, and (3) radioiodine ablation of the remaining lobe with subsequent radioactive iodine scanning.