The exact cause is still difficult to be sure, but from epidemiological investigations, experimental tumor studies and clinical observations, the occurrence of thyroid cancer may be related to the following factors 1.Radiation damage: irradiating the thyroid gland of experimental rats with X-rays can induce the development of thyroid cancer in animals. It is proved that 131Ⅰ can change the metabolism of thyroid cells, the nucleus is deformed and the synthesis of thyroxine is greatly reduced. It can be seen that on the one hand, radiation causes abnormal division of thyroid cells, leading to cancer; on the other hand, it destroys the thyroid gland and cannot produce endocrine hormone, and the resulting large secretion of thyroid stimulating hormone (TSH) can also promote thyroid cell carcinogenesis. 2.Iodine and TSH: Excessive iodine intake or iodine deficiency can change the structure and function of the thyroid gland. For example, the incidence of thyroid cancer in endemic areas of Switzerland is 20 times higher than that of non-endemic areas such as Berlin at 2 per 1,000. This may be related to the factor of TSH stimulating thyroid hyperplasia. Experiments have shown that long-term TSH stimulation can contribute to thyroid hyperplasia, nodule formation and carcinogenesis. 3. Other thyroid lesions: There are clinical reports of thyroid adenocarcinoma, chronic thyroiditis, nodular goiter or some toxic goiter becoming cancerous, but the relationship between these thyroid lesions and thyroid cancer is not yet certain. For example, most of the thyroid adenomas are follicular type and only 2-5% are papilloma; if thyroid cancer is transformed from adenoma, most of them should be follicular type, but in fact, more than half of the thyroid cancers are cephalic carcinoma, so it is assumed that the incidence of thyroid adenoma carcinoma is very small. Genetic factors: About 5-10% of medullary thyroid carcinomas have obvious family history and are often combined with pheochromocytoma, so it is presumed that the occurrence of such carcinomas may be related to chromosomal genetic factors.