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Fortunately, only about 5 to 10% of thyroid nodules are malignant (cancerous) and have the potential of invading tissue located near the thyroid gland and/or spreading to distant sites in the body. In general, there are four different types of malignant thyroid tumors:

• Papillary thyroid carcinoma
• Follicular thyroid carcinoma
• Anaplastic thyroid carcinoma
• Medullary thyroid carcinoma

Papillary Thyroid Carcinoma

Papillary thyroid carcinoma is the most common type of thyroid cancer representing about 65% to 85% of all cases. This type of cancer affects women more than men. Papillary thyroid carcinoma is typically a slow-growing tumor and in about 90% of cases only affects one lobe of the thyroid gland. In general, treatment for papillary thyroid carcinoma usually involves surgery to remove the thyroid gland (thyroidectomy), radioactive iodine therapy, and thyroid hormone replacement therapy.

Follicular Thyroid Carcinoma

Follicular thyroid carcinoma is the second most common type of thyroid cancer accounting for up to 15% of cases. This is a more aggressive form of thyroid cancer than papillary thyroid carcinoma and has a tendency to invade nearby tissue and/or spread to distant sites. In general, the treatment for follicular thyroid carcinoma is the same as that for papillary thyroid carcinoma.

Papillary thyroid carcinoma and follicular thyroid carcinoma are often grouped together and referred to in the medical literature as "differentiated thyroid cancer" or "well-differentiated thyroid cancer". Over 90% of all thyroid cancers are papillary and follicular thyroid carcinomas.

Anaplastic Thyroid Carcinoma

Anaplastic thyroid carcinoma is a very rare form of thyroid cancer representing only about 1% of all cases. Typically, this type of thyroid cancer occurs in elderly women. Anaplastic thyroid carcinoma is a very aggressive cancer that spreads to the neck and other areas of the body. This type of thyroid cancer is usually associated with a poor prognosis (outlook) with a 1-year survival rate of only about 20%. In general, treatment of anaplastic thyroid carcinoma involves surgery, chemotherapy, and radiation therapy.

Medullary Thyroid Carcinoma

Medullary thyroid carcinoma (MTC) is also a rare form of thyroid cancer and accounts for about 5% of all cases. Medullary thyroid carcinoma originates from the C cells of the thyroid gland that produce calcitonin. Consequently, the blood levels of calcitonin hormone are elevated in patients with MTC. This type of thyroid cancer tends to invade the lymphatic system and spread to other areas of the body.

Two distinct forms of MTC have been identified called sporadic MTC and familial MTC. Sporadic MTC is the most common form of MTC (80% of all cases) and does not run in families meaning that children do not inherit this cancer from their parents. Familial MTC, as the name implies, runs in families and a genetic test is available to screen children to see if they have inherited the disease. In some cases, patients with familial MTC may develop tumors in other endocrine glands such as the adrenal gland and parathyroid glands - a condition known as multiple endocrine neoplasia type 2 (MEN 2). In general, the treatment for medullary thyroid cancer is surgery to remove the thyroid gland. This type of thyroid cancer does not respond to radioactive iodine therapy. Additional surgery may be necessary if the cancer has spread to other organs.