In 2010, thyroid cancer was estimated to affect 44,670 individuals in the USA, according to data provided by the American Cancer Society; in the same year, 1,690 affected individuals died. The risk of developing thyroid cancer is approximately 1 in 286 or 0.35%. The risk of dying of thyroid cancer is even lower, approximately 1 in 2,500 or 0.04%. Differentiated thyroid cancer incidence is increasing annually at 3–5% per year. This increase is predominantly seen among the smallest thyroid cancers (those less than 2 cm); however, increases have been reported among the largest thyroid cancers as well. The reason for this increase is thought to be mainly due to increased diagnostic scrutiny and imaging for non-thyroid related ailments. However, the rise in incidence rates among the larger thyroid tumors suggests that other mechanisms may be at work.

Differentiated thyroid cancer comprises 90% of all thyroid cancers, and thus the discussion in this chapter will focus on these cancers. These cancers are pathologically defined as papillary and follicular thyroid cancer. The remaining 10% of thyroid cancers include medullary, anaplastic, poorly differentiated thyroid carcinoma, and lymphoma. Papillary thyroid cancers have a strong predilection to produce lymph node metastases. Follicular thyroid cancer tends to metastasize by hematogenous spread. Radiation exposure is the single highest risk factor in the development of thyroid cancer; however, more research has identified other molecular changes such as in the bRAF protein that leads to thyroid carcinogenesis. Approximately 45% of papillary thyroid cancers exhibit mutations of the bRAF protein. Research into the molecular changes necessary for development of differentiated thyroid cancer is ongoing.