Follicular thyroid cancer is the second most common thyroid malignancy after 
PTC. There are marked geographical variations in the relative proportions of FTC 
and PTC, most likely related to dietary iodine content. In iodine-deficient 
areas, the relative rate of FTC tends to be increased. Other risk factors for 
FTC include age over 50 years and female sex. Genetic factors may also have a 
role in determining disease susceptibility but remain ill-defined. 
Histologically, FTC is characterized by follicle formation and the absence of 
any papillary elements in the tumor. Differential diagnosis from a benign 
adenoma can be difficult. The degree of vascular invasiveness seems to correlate 
with tumor aggressiveness, and two histologic subtypes, oxyphilic FTC and 
insular FTC, may be associated with increased morbidity and mortality. Primary 
treatment for FTC is complete surgical tumor removal. Extensive bilateral 
surgery beyond this goal may not confer additional benefit but can facilitate 
adjuvant treatment and follow-up. Postoperative levothyroxine treatment is 
almost universally used, and patients deemed at high risk of recurrence may 
benefit from radioiodine remnant ablation. Treatment of metastatic disease 
involves operation, radioiodine, and, in selected cases, external beam radiation 
and chemotherapy. Prognosis for patients with metastatic disease is guarded, but 
most other patients have good outcomes comparable to that in PTC. For 
nonoxyphilic FTC, high-risk features other than initial metastases include 
advanced age, locally extensive disease, and the presence of marked 
angioinvasion. In oxyphilic FTC, DNA aneuploidy is also important. Follow-up 
should be most intense during the first 5 years after primary treatment and 
needs to be tailored to the patient's risk of disease progression. For patients 
at low risk of recurrence (young, small lesions, minimally invasive tumor), 
serum thyroglobulin measurements may largely suffice, whereas higher risk 
patients with elevated serum thyroglobulin levels and patients with significant 
titers of interfering anti-thyroglobulin autoantibodies may also need to undergo 
periodic diagnostic radioiodine scanning.