The thyroid is a small, butterfly-shaped gland located in the front of the neck below the larynx (voice box). The thyroid is part of the endocrine system, which regulates hormones. The thyroid contains follicular cells, which use iodine — a mineral found in some foods and in iodized salt — to make hormones that control the body’s heart rate, blood pressure, temperature, and the rate at which food is turned into energy (metabolism). The thyroid also contains C cells, which make the hormone calcitonin.
Thyroid cancer begins when cells in the thyroid gland begin to change and grow uncontrollably, forming a nodule (tumor). Ninety percent of all thyroid nodules are benign (noncancerous), but a few are malignant (cancerous).
An estimated 37,200 new cases of thyroid cancer are diagnosed each year in the United States. Women are nearly three times more likely to develop a thyroid tumor than men. The chance of diagnosis has risen considerably in recent years, due in part to improved detection of small tumors with ultrasound, PET, and CT imaging1. Unlike many other types of cancer, which are more common in older people, thyroid cancer occurs mainly in adults between the ages 20 and 55. It is also one of the most curable types of cancer — approximately 95 percent of people with a thyroid tumor survive at least five years beyond their diagnosis.
Differentiated Thyroid Cancers
Differentiated thyroid cancers, which begin in the follicular cells of the thyroid, contain cells that look similar to those in normal thyroid tissue when examined under a microscope. There are three types of differentiated thyroid cancers:
The vast majority of all thyroid cancers are papillary carcinoma, a slow-growing tumor that usually develops in one lobe of the thyroid gland. Although papillary carcinoma can spread to the lymph nodes in the neck, most people with this type of cancer can be cured.
A less common type of thyroid cancer, follicular carcinoma grows slowly and can often be cured. Follicular carcinoma rarely spreads to the lymph nodes, although it can spread to other parts of the body, such as the lungs and bones.
Hurthle Cell Carcinoma
Hurthle cell carcinoma (also called oxyphil cell carcinoma) is a rare type of follicular carcinoma. Hurthle cell tumors that are not cured with surgery can be more difficult to treat than other types of differentiated thyroid cancers because they absorb less radioactive iodine than papillary and follicular thyroid tumors.
Other Types of Thyroid Cancer
Less common types of thyroid cancer include the following:
Medullary Thyroid Carcinoma (MTC)
This rare type of thyroid cancer begins in the C cells, which make the hormone calcitonin. High levels of calcitonin and a protein called carcinoembryonic antigen (CEA) are a common indication of MTC. MTC usually occurs in older adults and often affects only one lobe of the thyroid. Some patients have familial MTC, an inherited form of thyroid cancer that develops during childhood or early adulthood and in several areas of each lobe.
Also called undifferentiated carcinoma, anaplastic carcinoma usually develops from an existing papillary or follicular carcinoma. This aggressive type of cancer, which spreads rapidly throughout the neck and to other parts of the body, is very difficult to control.
The following factors can increase a person’s risk of developing thyroid cancer:
In the United States, children who received moderate levels of radiation during the 1950s and 1960s — from low- to moderate-dose x-rays used to treat acne, tonsillitis, and other head and neck problems, or from radiation therapy used to treat Hodgkin lymphoma — have a higher risk of developing papillary and follicular thyroid cancers. In Eastern Europe, there is a significantly higher incidence of thyroid cancer among people who were exposed to excess radiation from the Chernobyl nuclear disaster as children and adolescents.
Follicular carcinoma is more common in places where iodine is not added to salt. A low-iodine diet combined with radiation exposure further increases the risk of thyroid cancer.
Family History of Thyroid Cancer
Approximately 25 percent of patients with medullary thyroid cancer have a mutation in a gene called RET. Memorial Sloan Kettering offers genetic screening and testing for family members of patients with the mutated gene. For more information, visit Hereditary Cancer & Genetics. Papillary thyroid cancer is not usually hereditary.
Family History of Colon Growths
Papillary carcinoma may also occur in some people with a family history of multiple growths in the colon, such as in a condition called familial adenomatous polyposis (FAP).
For unknown reasons, papillary thyroid cancer occurs about three times more often in women than in men. However, this type of cancer is more aggressive in men.
Other Genetic Risk Factors
Some rare genetic diseases, such as Cowden disease, are also associated with a higher risk of thyroid cancer.
Many patients with thyroid cancer have no noticeable symptoms. When they do occur, swelling or a small nodule (lump) on the front of the neck is often the first symptom. Thyroid nodules are common and are usually painless. The vast majority of them are benign.
Other less common symptoms of thyroid cancer may include:
- trouble swallowing
- persistently swollen glands in the neck
- difficulty breathing
- pain in the throat or neck that does not go away
- a cough that does not go away and is not due to a cold
Symptoms of thyroid cancer may be similar to those of other medical conditions, such as a goiter or an infection. If you are concerned about a symptom on this list, please talk with your doctor.
L. Davies, H. G. Welch. Increasing incidence of thyroid cancer in the United States, 1973-2002. JAMA. 2006 May 10;295(18):2164-7. [PubMed Abstract]