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INTRODUCTION

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Thyroid nodules are a commonly encountered clinical entity. Although the majority of thyroid nodules are benign, the risk of malignancy necessitates a thorough evaluation and workup. Evaluation of thyroid nodules is aimed at achieving diagnosis and determining subsequent management: benign asymptomatic nodules undergo monitoring, cytologically abnormal nodules lead to surgical referral for definitive therapy of malignancy or definitive diagnosis of indeterminate nodules, and symptomatic nodular goiters are also candidates for surgery.

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Definition

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In 2009, the American Thyroid Association released guidelines for the management of patients with thyroid nodules, which defined a thyroid nodule as “a discrete lesion within the thyroid gland that is radiologically distinct from the surrounding thyroid parenchyma.”1 Palpable lesions may be appreciated within the thyroid on physical exam, but if a radiologic abnormality is not present, these lesions are not classified as thyroid nodules. Nonpalpable lesions may also be identified on imaging. Such unsuspected, asymptomatic thyroid lesions discovered on imaging or during an operation unrelated to the thyroid gland are termed “incidentalomas” and require workup according to the same guidelines as palpable thyroid nodules.2,3

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Epidemiology

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Thyroid nodules are common, with a reported incidence of 0.1% per year and prevalence of 3% to 7% by palpation on physical exam.4,5 Anatomic imaging techniques, including ultrasound, detect nodules at much higher rates. Prevalence rates of incidental thyroid nodules found on ultrasound are reported to be 20% to 76% in the adult population4,6,7 which correlates with the reported prevalence of 30% to 60% for unsuspected nodules in autopsy series.5,6 Twenty percent to 48% of patients with a single palpable nodule are found to have additional nodules on ultrasound evaluation;4,7 likewise, those with one nodule found on ultrasound, but not appreciated on exam, are frequently found to have multiple additional nodules during the ultrasound examination.7 It is estimated that approximately 500,000 thyroid nodule fine-needle aspiration (FNA) biopsies are performed annually in the United States.8

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Risk Factors

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Risk factors for the development of thyroid nodules include gender, age, iodine intake, and radiation exposure. Women are approximately four times more likely than men to have both palpable and incidentally discovered thyroid nodules.1,9 Older age and low iodine intake also confer an increased risk of thyroid nodule development. Exposure to ionizing radiation of 2 to 5 Gy, especially as a child, is associated with 2% annual risk of the development of thyroid nodules, with a peak incidence approximately 20 years after the exposure.9

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The risk of malignancy in asymptomatic nodules is approximately 5%.5 The risk of malignancy does not vary significantly between those with a solitary nodule and those with multinodular goiter.1,4 Clinical findings, however, that should increase the clinician’s concern for malignancy include the ...

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