Section 13. Hormonal Disorders

Chapter 163. Thyroid Gland Disorders

Topics: Introduction | Hyperthyroidism | Hypothyroidism | Thyroiditis | Thyroid Cancer

Thyroid Cancer

The cause of thyroid cancer is not known, but the thyroid gland is very sensitive to radiation. 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).

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). Nodules are more likely to be cancerous if only one nodule is found rather than several, if the nodule is solid rather than filled with fluid (cystic), if the nodule is not producing much thyroid hormone, if the nodule is hard, if the nodule is growing quickly, or if the nodule 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 test is generally a measurement of the level of thyroid-stimulating hormone (TSH) in the blood to determine if hyperthyroidism is present. A thyroid scan determines whether the nodule is producing thyroid hormone and is done when the level of TSH is low. An ultrasound scan is less helpful but may be performed to determine whether the nodule is solid or filled with fluid or whether other nodules are present.

A fine-needle biopsy, in which a sample of the nodule is removed through a small needle and then examined under a microscope, is usually performed. This procedure is almost always painless, is carried out in the doctor's office, and may involve the use of a local anesthetic as well as ultrasound to guide needle placement.

Cancer can develop in any of several types of cells within the thyroid gland.

Papillary cancer is the most common type, accounting for 60 to 70% 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 ¾ inch are removed along with the thyroid tissue immediately surrounding them, although some 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 accounts for about 15% 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 often curable, but less so than papillary cancer.

Anaplastic cancer accounts for less than 5% 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 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 Section 13, Chapter 167).

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. When medullary thyroid cancer occurs alone, the chances of survival are not as good.