1. Epidemiology and Etiology of Thyroid Nodules and Thyroid Cancers

Thyroid nodules are very common but thyroid cancers are not. The data that follow relate predominantly to the United States (US). Between 5% to 7% of adults have a clinically detectable nodule in the thyroid and 30% to 50% of adults have one or more nodules in the thyroid when the gland is examined by ultrasound. Therefore, in the adult population, approximately 10 and 10 have thyroid nodules that are palpable, or ultrasonically visible. In contrast, there are approximately 30,000 new cases of thyroid cancer annually in the US. Physicians should have an algorithm for management of nodules to identify the small proportion that are malignant from the very large proportion that are benign (Chapter 4). Thyroid nodules are more common in women and in regions of low intake of iodine. External radiation increases the incidence. There are families in which there is an increased incidence of clinically palpable nodules and some of these are part of syndromes such as Cowden’s syndrome. There are also familial aggregations of medullary thyroid cancer and occasionally papillary cancer.