Although scientists have found that thyroid cancer is linked with a number of other conditions, the exact cause of most thyroid cancers is not yet known.
Researchers have made great progress in understanding how certain changes in a person's DNA can cause thyroid cells to become cancerous. DNA is the chemical in each of our cells that makes up our genes -- the instructions for how our cells function. We usually resemble our parents because they are the source of our DNA. However, DNA affects more than how we look. It also can influence our risk for developing certain diseases, including some kinds of cancer.
Some genes contain instructions for controlling when our cells grow and divide. Certain genes that speed up cell division or cause cells to live longer than they should are called oncogenes. Others that slow down cell division or cause cells to die at the appropriate time are called tumor suppressor genes. Cancers can be caused by DNA mutations (defects) that turn on oncogenes or turn off tumor suppressor genes.
People inherit 2 copies of each gene -- one from each parent.
People can inherit damaged DNA from one or both parents, which accounts for inherited cancers. Most cancers, though, are not inherited. In these cases, a person's DNA is damaged by exposure to something in the environment, like smoking or radiation. Sometimes DNA mutates for no apparent reason.
Papillary Thyroid Cancer: Several DNA mutations have been found in some forms of papillary thyroid cancer. Many of these cancers have changes in specific parts of the RET gene. The altered form of this gene, known as the PTC oncogene, is found in about 10% to 30% of papillary thyroid cancers overall, and in a larger percentage of these cancers found in children and/or linked with radiation exposure. These RET mutations usually are acquired during a person's lifetime rather than being inherited. They are present only in the cancer cell and are not passed on to the patient's children.
- Many (30% to 70%) papillary thyroid cancers contain a mutation of the BRAF gene. The BRAF mutation is less common in thyroid cancers in children and in those thought to arise from exposure to radiation. Cancers with BRAF changes tend to have more aggressive growth and a greater likelihood of spreading to other parts of the body.
- Both BRAF and RET/PTC changes are thought to cause cells to grow and divide. It is extremely rare for papillary cancers to have changes in both the BRAF and RET/PTC genes.
- Changes to other genes have also been tied to papillary thyroid cancer, including those in the NTRK1 gene and the MET gene.
Follicular Thyroid Cancer: Acquired changes in the RAS oncogene have a role in causing some follicular thyroid cancers.
Anaplastic Thyroid Cancer: These cancers tend to have some of the mutations described above and often have changes in the p53 tumor suppressor gene as well. This gene is the most commonly mutated gene in human cancers and is not specifically associated with anaplastic thyroid cancer.
Medullary Thyroid Cancer: People who have medullary thyroid carcinoma (MTC) have mutations in different parts of the RET gene compared with papillary carcinoma patients. Nearly all patients with the inherited form of MTC and about 1 of every 10 with the sporadic (non-inherited) form of MTC have a mutation in the RET gene.
Most patients with sporadic MTC have acquired mutations present only in their cancer cells. Those with familial MTC and MEN 2 inherit the RET mutation from a parent. These mutations are present in every cell of the patient's body and can be detected by testing the DNA of blood cells.
In people with inherited mutations of RET, one RET gene is usually normal and one is mutated. Because every person has 2 RET genes but passes only one of them to a child (the child's other RET gene comes from the other parent), the odds that a person with familial MTC will pass a mutated gene on to a child are 1 in 2 (or 50%).