The exact cause of most thyroid cancers is unknown. Some forms of cancer are caused by DNA mutations that “turn on” oncogenes (genes that speed up cell division) or “turn off” tumor suppressor genes (genes that slow down cell division or cause cells to die at the right time). Some people with thyroid cancer inherit these mutations but more commonly they are acquired during the patient’s lifetime.

Patients with mutations in the RET proto-oncogene (RET gene) have an increased risk of developing many forms of thyroid cancer. The function of the RET gene is to produce proteins needed for the development of several kinds of nerve cells. Mutations in the RET gene can cause:

• Papillary carcinoma. The DNA mutations that trigger some forms of papillary carcinoma are known to involve particular parts of the RET oncogene. Found only in the cancer cells, these mutations are acquired during a patient’s lifetime, and are not passed on to the patient’s children.
  
  
• Medullary thyroid carcinoma (MTC). Nearly all people with familial medullary thyroid carcinoma (FMTC), and about one in every five patients with sporadic medullary thyroid carcinoma have a mutation in the RET gene. Most people with sporadic MTC have acquired mutations found only in their cancer cells. These mutations are not passed on to children. Patients with FMTC and MEN 2 inherit the mutation in the RET gene from a parent. The mutations are found in every cell and can be detected by genetic blood testing. According to the American Cancer Society (ACS), there is a 50 percent chance that a parent with FMTC will pass a mutated gene to a child. 

Acquired changes in other oncogenes and tumor suppressor genes (e.g., RAS, TRK, GSP, and p53) also have a role in causing papillary carcinoma and follicular carcinoma. Although these mutations have been identified and connected with thyroid cancer, researchers are still unaware of exactly how they occur and why they develop in only certain people.  

Although there is no single cause of thyroid cancer, researchers have identified a number of factors that appear to place a person at higher risk for developing the disease. These risk factors include:

• Diet low in iodine. Papillary and follicular thyroid carcinomas are more common in populations where diets are low in iodine. In the United States, dietary iodine is plentiful because it is added to table salt and other foods.
  
  
• Exposure to radiation. People who received radiation treatments to the head or neck during childhood have an increased risk of developing papillary carcinoma. Radiation was previously used to treat a number of conditions, such as scalp ringworm and enlarged thymus or tonsils. The cancer may develop as early as five years after the treatment, or after 20 or more years.  Exposure to radiation treatments as an adult poses little risk for thyroid cancer. In addition, studies have suggested that exposure to very high levels of radioactive material, such as from atomic weapons or nuclear power plant accidents, may increase a person’s risk of developing the disease.  
  
  
• Genetic conditions. People with certain inherited conditions are at a greater risk of developing thyroid cancer. These conditions include:
  
  
• • Gardner’s syndrome. A hereditary disorder in which many polyps form in the digestive tract.
    
    
  • Familial polyposis. A hereditary disorder in which many polyps form in the colon.
    
    
  • Cowden’s disease. A rare hereditary disorder in which lesions form on the face, hands and feet or inside of the mouth.
    
    
  Approximately 20 percent of medullary thyroid carcinomas (MTC) result from the inheritance of an abnormal gene.
  
  
• Goiter. People with enlargement of the thyroid gland (a goiter) have an increased risk of developing thyroid cancer.
  
  
• Family history of thyroid disease. People who have close relatives with any form of thyroid disease have an increased risk of developing thyroid cancer. 
  
  
• Gender. Thyroid cancer occurs more often in women.
  
  
• Age. In general, thyroid cancer occurs more often in people between the ages of 25 and 65. Most cases of papillary carcinoma and follicular carcinoma occur in people between the ages of 20 and 55.