The thyroid gland is part of the endocrine system, and its function is to produce hormones that help regulate the body’s metabolism. In the thyroid, healthy cells normally grow, divide and then cease to divide in a systematic way. Cancer of the thyroid has its start when cells lose their ability to stop reproducing then die as they normally would but, instead, keep dividing and multiplying when they are no longer needed, eventually forming a tumor. As of early 2011, medical researchers had not established precise causes of thyroid cancer. Genes are recognized as playing a central role as one of the causes of thyroid cancer, but independent risk factors such as radiation, age and sex have also been found to be associated with the development of thyroid cancer, so the relative importance of genes versus risk factors as causes of thyroid cancer has remained unclear to scientists.
There are four major types of thyroid cancer: papillary, follicular, anaplastic and medullary. Papillary and follicular are by far the most common types of thyroid cancer. In the case of these forms, medical researchers have been able to establish that gene mutations are present in a significant percentage of these cancers. For both the anaplastic and medullary forms of thyroid cancer, a genetic connection also has been established, but in a lower percentage of cases.
In addition to genetics, medical researchers have identified several independent risk factors as causes of thyroid cancer. The most common risk factors among the causes of thyroid cancer are exposure to radiation, family history and lack of iodine. Gender, age and race are also known risk factors.
Exposure to high levels of radiation is a proven risk factor for thyroid cancer. Radiation treatments to the head and neck for medical reasons are one source of individuals receiving high doses of radiation. Fallout from the testing and use of nuclear weapons is another. Finally, nuclear power plant accidents also might cause exposure to radiation. Exposure to radiation as a child carries more risk than exposure during adulthood.
A family history of goiter, which is a benign enlargement of the thyroid gland, is a risk factor for thyroid cancer. Certain inherited conditions, such as Cowden disease, also increase the risk. Some forms of thyroid cancer run in families because of defective genes that are inherited. Diets that are too low in iodine are risk factors for papillary and follicular thyroid cancer. Lastly, women are more likely to develop thyroid cancer, as are people who are more than 40 years old and, in the United States, those who are Caucasian.