Iodine deficiency leads to the benign overgrowth of the thyroid to produce ‘goitres’ and this has led clinicians to study the relationship between environmental iodine availability and thyroid cancer. It has been demonstrated that follicular carcinoma (see below) is more common in regions of low environmental iodine whereas papillary carcinoma (see below) is as common or even more common in iodine avid as deprived regions.
Radioactive iodine pollution in the atmosphere is probably more carcinogenic and fall-out from nuclear accidents, bombs, or emissions from power stations are risks for the later development of thyroid cancer. Indeed, after exposure to nuclear fall-out containing radioactive iodine it is recommended to ingest ‘cold’ (non-radioactive) iodine to swamp the thyroid and so dilute the amount of radioactive isotope concentrated by the gland for this reason.
Total body exposure to ionising radiation of whatever source is also predisposing to thyroid cancer after an interval of some years.
Thyroid cancer is considerably more common in females than it is in males in all parts of the world, although the ratio female:male varies from 2:1 to 4:1.
There is a small incidence in childhood and then a significant incidence in young adult women before the incidence slowly and progressively rises as age increases.
There are a few very rare hereditary causes of thyroid cancer and medullary thyroid cancer (see below) occurs in the multiple endocrine neoplasia syndrome along with other primary tumours.
Thyroid lymphoma tends to occur in the elderly who have suffered autoimmune thyroiditis for a long time previous to the development of the lymphoma, and the inference is that the immune lymphocytes invading the gland in the benign thyroiditis have eventually turned malignant to become a lymphoma.
Anaplastic thyroid cancer (see below) is also a disease that tends to occur in the elderly; it is not an iodine avid cancer and therefore radioactive iodine therapy has no role (as indeed it does not in neither thyroid lymphoma nor medullary cancer).