The cause of thyroid cancer is not known, but the thyroid gland is very sensitive to radiation, which may cause cancerous changes. Thyroid cancer is more common among people who were treated with radiation to the head, neck, or chest, most often for noncancerous (benign) conditions, when they were children (although radiation treatment for noncancerous conditions is no longer used).

Thyroid nodules: Rather than causing the whole thyroid gland to enlarge, a cancer usually causes small growths (nodules) to develop within the thyroid. However, most thyroid nodules are not cancerous (malignant). A nodule is more likely to be cancerous if it

• Is solid rather than filled with fluid (cystic)
• Is not producing thyroid hormone
• Is hard
• Is growing quickly
• Occurs in a man

A painless lump in the neck is usually the first sign of thyroid cancer. When doctors find a nodule in the thyroid gland, they request several tests. The first tests are generally measurement of the blood levels of thyroid-stimulating hormone (TSH), thyroid hormones T₄ (thyroxine, or tetraiodothyronine) and T₃ (triiodothyronine), and sometimes thyroid antibodies. If these tests show hyperthyroidism is present, a thyroid scan is done to determine whether the nodule is producing thyroid hormones. Nodules that are producing hormones ("hot" nodules) are almost never cancerous. If the tests do not indicate hyperthyroidism or Hashimoto's thyroiditis, or if the nodules are not “hot,” doctors usually do a fine-needle biopsy. In a fine-needle biopsy, a sample of the nodule is removed through a small needle and then examined under a microscope. This procedure is not very painful, is carried out in the doctor's office, and may involve the use of a local anesthetic as well as ultrasonography to guide needle placement. Ultrasonography may be done to determine how large the nodule is, whether it is solid or filled with fluid, and whether other nodules are present.

Types of Cancer

Papillary cancer: Papillary cancer is the most common type, accounting for 80% of all thyroid cancers. About 2 to 3 times as many women as men have papillary cancer. Papillary cancer is more common in young people but grows and spreads more quickly in older people. People who have received radiation treatment to the neck, usually for a noncancerous condition in infancy or childhood or for some other cancer in adulthood, are at greater risk of developing papillary cancer.

Papillary cancer grows within the thyroid gland but sometimes spreads (metastasizes) to nearby lymph nodes. If left untreated, papillary cancer may spread to more distant sites.

Papillary cancer is almost always curable. Nodules smaller than about a half inch (1 centimeter) are removed along with the thyroid tissue immediately surrounding them, although many experts recommend removing the entire thyroid gland. For larger nodules, most or all of the thyroid gland is usually removed. Radioactive iodine is often given to destroy any remaining thyroid tissue or cancer. Thyroid hormone is also given in large doses to suppress the growth of any remaining thyroid tissue.

Follicular cancer: Follicular cancer accounts for about 10 to15% of all thyroid cancers and is more common among older people. Follicular cancer is also more common in women than in men.

Much more aggressive than papillary cancer, follicular cancer tends to spread (metastasize) through the bloodstream, spreading cancerous cells to various parts of the body.

Treatment for follicular cancer requires surgically removing as much of the thyroid gland as possible and destroying any remaining thyroid tissue, including the metastases, if present, with radioactive iodine. It is usually curable, but less so than papillary cancer.

Anaplastic cancer: Anaplastic cancer accounts for about 1% of thyroid cancers and is most common among older women. This cancer grows very quickly and usually causes a large growth in the neck. It also tends to spread throughout the body.

About 80% of people with anaplastic cancer die within 1 year, even with treatment. However, treatment with chemotherapy and radiation therapy before and after surgery has resulted in some cures. Radioactive iodine is not helpful in the treatment of this type of cancer.

Medullary cancer: About 5% of thyroid cancers are a medullary carcinoma that begins in the thyroid gland but in a different type of cell than that which produces thyroid hormone. The origin of this cancer is the C-cell, which is normally dispersed throughout the thyroid and secretes the hormone calcitonin. The cancer produces excessive amounts of calcitonin. Because medullary thyroid cancer can also produce other hormones, it can cause unusual symptoms.

This cancer tends to spread (metastasize) through the lymphatic vessels to the lymph nodes and through the blood to the liver, lungs, and bones. Medullary cancer can develop along with other types of endocrine cancers in what is called multiple endocrine neoplasia syndrome (see Multiple Endocrine Neoplasia Syndromes).

Treatment requires surgically removing the thyroid gland. Additional surgery may be needed to determine whether the cancer has spread to the lymph nodes. More than two thirds of people whose medullary thyroid cancer is part of multiple endocrine neoplasia syndrome are cured.

Last full review/revision May 2012 by Jerome M. Hershman, MD