Thyroid nodules (abnormal lumps in the neck) can be either benign (noncancerous) or malignant (cancerous). Approximately 5 to 10 percent of thyroid nodules are malignant. The four main types of malignant thyroid tumors include:

• Papillary carcinoma. Also known as papillary cancer or papillary adenocarcinoma, these tumors evolve from the thyroid follicle cells and typically grow very slowly. Papillary carcinomas account for approximately 70 to 80 percent of thyroid cancers. This cancer usually occurs in only one lobe of the thyroid gland. Both lobes are affected in about 10 percent of cases. Several subtypes of papillary carcinoma can be identified under a microscope. Treatment decisions and prognosis depend on the form of the disease. Although papillary carcinoma frequently spreads early to the lymph nodes in the neck, this form of cancer has a very high survival rate.
  
  

  

• Follicular carcinoma. This cancer normally remains in the thyroid gland, but can spread to other areas of the body including the lungs and bone. It is the second most common form of thyroid cancer, accounting for approximately 20 percent of all cases. Also known as follicular cancer or follicular adenocarcinoma, the disease is more common in areas where people have iodine-deficient diets.
  
  
• Medullary thyroid carcinoma (MTC). MTC accounts for approximately 3 percent of thyroid cancers. It is the only form of thyroid cancer that evolves from the C cells of the thyroid gland. One of the two types of MTC is familial medullary thyroid carcinoma (FMTC), an inherited cancer. FMTC may occur in people who have a condition called type 2 multiple endocrine neoplasia (MEN 2), which can produce tumors in other endocrine glands.
  
  
• Anaplastic carcinoma. The rarest form of thyroid cancer, anaplastic carcinoma is believed to evolve from an existing papillary or follicular cancer. Also known as undifferentiated thyroid cancer, anaplastic carcinoma is an aggressive cancer that quickly invades the neck, and often spreads to other areas of the body. It is usually fatal. When examined under a microscope, its cells have very little resemblance to healthy thyroid cells.