The reasons for the development of thyroid cancer are not clearly understood.

However a family history of thyroid cancer and/or radiation exposure, especially in childhood, is associated with an increased risk. 

X-ray irradiation of the head and neck region - especially during childhood and adolescence – increases the risk of developing thyroid cancer. People who have been exposed to high levels of radiation, e.g. following a nuclear reactor disaster, are also at increased risk of developing thyroid cancer. In this situation, the risk of cancer increases with higher doses of radiation exposure, with children and adolescents being particularly vulnerable.

Iodine deficiency over a long period can increase the risk of developing thyroid cancer. Patients who suffer from an enlarged thyroid ("goitre") due to iodine deficiency particularly over a long period, have an increased risk of developing thyroid cancer.

Causes of medullary thyroid cancer

Medullary thyroid cancer is inherited in 25-30% of cases and it occurs sporadically (by chance) in 75-80% of cases. Genetic changes are associated with the development of medullary thyroid cancer but the underlying cause of genetic mutations is unknown.