Evaluation of the thyroid nodule.

Thyroid nodules are very common in the general population. The prevalence of palpable thyroid nodules is only approximately 4–7%, but the prevalence of ultrasound-detectable nodules is between 19 and 67% [1]. Twenty to forty-eight percent of patients with apparently solitary thyroid nodules palpated on physical exam will have additional sonographically detectable nodules [2, 3]. Thyroid nodules are more common in women than in men by a ratio of about 4 to 1, and increase in frequency with age and with decreasing iodine intake [4]. Thyroid nodules are also more common in patients who have a history of head and neck irradiation, developing at a rate of about 2% per year compared with 0.1% per year in patients without a history of significant radiation exposure [5]. The great majority of thyroid nodules are benign, with the differential diagnosis including simple or hemorrhagic cysts, colloid nodules, follicular adenomas, or thyroiditis [6]. The overall risk of malignancy in a thyroid nodule is 5–10% [7]. While thyroid nodules are relatively common in the general population, in contrast, thyroid cancer is relatively uncommon with an expected annual incidence in the United States (U.S.) of 37,340 cases in 2008, constituting only 2.6% of all cancers and only 0.3% of cancer deaths [8]. The incidence of thyroid cancer has increased 2.4-fold in the U.S. over the last 30 years, from 3.6 per 100,000 in 1973 to 8.7 per 100,000 in 2003 [9]. Thyroid cancer is more common in women than in men by a ratio of about 3 to 1, and has now become the sixth most common cancer in women [8]. While thyroid cancer is more common in women, mortality rates from thyroid cancer are higher for men; this is thought to be related to an older age at diagnosis in men [10].