Thyroid cancer is a fairly common malignancy. Thyroid carcinomas (cancers) are a heterogeneous group of tumours that demonstrates considerable variability in behaviour, appearance and response to therapy. Although benign thyroid nodules are common, thyroid carcinoma is rare representing approximately 1% of all malignancies and 2% of all cancer deaths.
Recognized types of thyroid cancer and their frequency are are (1) papillary and mixed papillary/follicular, 75%; (2) follicular and Hurthle cell, 15% (3) medullary, 7%; and (4) anaplastic, 3%.
Thyroid carcinoma occurs with an incidence of approximately 36 to 60 cases per million population per year, ie up to 0.006% per year. These tumours are rare in children and increase in frequency with increasing age. A female to male ratio of 2.5:1 is reported. Incidence in Bombay is 0.4/100,000 in males and 1.0 per 100,000 in females.
Since 1975 in the USA, the incidence of thyroid cancer in women has more than tripled from 6.5 to 21.4 per 100,000 women, mostly from papillary cancer and mostly in younger women. Annual mortality from thyroid carcinoma in the United States is only 6 per million population and this has remained stable even though the incidence hastripled. This discrepancy between incidence and mortality has been interpeted to reflect the favorable prognosis for most thyroid carcinomas; alternatively an overdiagnosis of thyroid tumours as cancer rather than benign growths.
Many thyroid cancers are really not cancer and should be reclassified. An example is the encapsulated follicular variant of papillary thyroid carcinoma. These cases are treated as having conventional thyroid cancer, yet they are not really cancer.
Increased detection of thyroid cancer results in surgery and radioactive thyroid treatment that may be of limited benefit. The autopsy prevalence rate of occult thyroid cancer in the Finnish population is 35.6%, suggesting that thyroid cancer is both common and clinically insignificant for the vast majority of individuals. It is likely that the majority of diagnosed thyroid cancer patients will not benefit from surgical and/or adjuvant interventions.
There is an ongoing epidemic of thyroid cancer in the United States. The epidemiology of the increased incidence, however, suggests that it is not an epidemic of disease but rather an epidemic of diagnosis. The problem is particularly acute for women, who have lower autopsy prevalence of thyroid cancer than men but higher cancer detection rates.