Why does Thyroid Cancer occur?

Thyroid cancer is rare; there are about  one thousand six hundred newly diagnosed cases a year in the United Kingdom. Usually the cause is not known, however there are four predisposing factors:

Radiation

It is well documented that radiation to the neck given to children for non-cancerous skin conditions such as acne or tuberculosis results years later in the development of thyroid cancers. Nuclear accidents such as Chernobyl result in an increased incidence of thyroid cancer in the affected population.

Genetic

Medullary cancer of the thyroid may arise as a familial condition due to a variety of germline mutations in the RET gene. In addition there is a genetic mutation that causes familial papillary thyroid cancer.

Iodine

Follicular cancer rarely occurs in Iceland where, due to high dietary intake of fish, there is high iodide intake. In contrast, in areas of low iodide intake (mountainous regions such as the Alps, Andes and Himalayas) follicular cancer has a high incidence. Iodising salt so as to combat iodine deficiency does not change the incidence of thyroid cancer but increases the proportion of papillary cancers and reduces the incidence of follicular cancer. The overall incidence therefore remains virtually the same.

Thyroiditis

Papillary cancers may arise more often in patients with a long-term severe inflammation of their thyroid (thyroiditis). A special form of thyroiditis (Hashimoto's Disease) may develop into a form of cancer called lymphoma.

Lateral Aberrant Thyroid

In the past the presence of normal thyroid tissue even within the lateral neck was considered of no significance and was called "lateral aberrant thyroid". This concept is now considered incorrect and thyroid tissue in the lateral neck, if within lymph nodes, is now thought to represent metastatic papillary cancer.

In many cases the primary is not obvious in the thyroid even on high quality ultrasound. The primary may be microscopic and only found by meticulous sectioning of the thyroid. The prognosis of this type of cancer is excellent and in most cases, despite the fact that the thyroid appears to be normal, the treatment is total thyroidectomy with a neck dissection.

Laterally placed true non metastatic thyroid tissue can occur if it is not within lymph nodes and is medial to the carotid vessels. Neck surgery for thyroidectomy can result in implantation of thyroid tissue quite lateral in the neck and such thyroid tissue is never within cervical lymph nodes.