In most cases, the etiology for thyroid cancer is unknown.
In 20-30% of patients with medullary thyroid carcinoma (MTC), the disease is associated with genetic syndromes (mutation in the RET oncogene), which causes familial MTC. Some of these patients also develop disturbances in other hormone producing glands (multiple endocrine neoplasias (MEN)). Papillary thyroid carcinoma (PTC) can, in rare cases, also be familial.
Patients who have received external radiation therapy to the neck have an increased risk of developing thyroid cancer.
Radiation may be a contributing factor in PTC. An example of this is the Chernobyl disaster. During the years after the catastrophy, the incidence of PTC among children and youth in the areas around Chernobyl increased. An equivalent increase has not been registered in Norway.
At the end of the seventies, it was discovered that the incidence of follicular carcinomas (FTC) was highest in areas of low iodine intake. Later research showed that addition of iodine in food reduced the risk of FTC, but the number of PTC increased in the same areas. Iodine supplementation has become more common in the western countries, and because the histopathological definition of FTC has become more specific, the collective incidence has decreased.
There is also a correlation between thyroiditis and PTC.