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Thyroid Incidentalomas: Management Approaches To Nonpalpable Nodules Discovered Incidentally On Thyroid Imaging

G. H. Tan, H. Gharib
Published 1997 · Medicine

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In North America, the incidence of thyroid nodules detected by palpation is estimated to be 0.1% per year, with a prevalence between 4% and 7% in the general population. Thyroid nodules are more common in women, with advancing age, in areas of iodine deficiency, and after exposure to external radiation [1]. Most thyroid nodules are benign, and nonsurgical diagnostic approaches to the nodules are currently preferred. Fine-needle aspiration biopsy has emerged as the most accurate diagnostic test for differentiating benign from malignant thyroid nodules. Benign nodules are treated medically, but those with a high risk for malignancy are selected for surgical resection [1]. During the past decade, improved technology has increased the sensitivity of many imaging devices, resulting in the discovery of subclinical nodules in the adrenal, pituitary, and thyroid glands [2-4]. The increasing use of sensitive, high-frequency ultrasonography has led to the identification of nonpalpable thyroid nodules during nonthyroidal ultrasonographic examination of the neck [2]. The discovery of one or more nodules within an otherwise clinically normal thyroid gland raises concern about malignancy and creates a difficult treatment decision for clinician and patient. These lesions, which are referred to as incidentalomas, are small and nonpalpable and are incidentally discovered on ultrasonography. Although most authorities recommend fine-needle aspiration biopsy for palpable nodules, the optimal method for treating nonpalpable nodules is a matter of controversy. We review the frequency and clinical significance of incidentally discovered, nonpalpable thyroid nodules and offer a practical approach to their treatment. Data Sources We reviewed relevant articles published in major English-language medical journals during the past 15 years. We included prospective and retrospective studies that evaluated the prevalence of incidental thyroid nodules as determined by neck ultrasonography, by other imaging studies for thyroid and nonthyroid diseases, and by autopsy studies. Search Strategy The MEDLINE and Current Contents, Sciences Edition, computerized databases were used to search the medical literature published in the past 15 years. We used the keywords thyroid nodule and goiter, nodular, and we did keyword and textword searches using the terms occult, incidental, impalpable, and unexpected. We obtained the full text of the articles that met our criteria. We also reviewed articles cited in the articles identified in our database searches. To formulate our algorithm for approaching thyroid incidentalomas, we also searched using the textword thyroid neoplasms in combination with the previous textword searches. Data Extraction We examined each article identified in our search and determined which were eligible. A total of 135 articles and abstracts were reviewed. Three articles reported autopsy findings, and 11 reported ultrasonographic findings, either in comparison with findings on clinical palpation or as part of prospective studies in a given population. One study compared scintigraphic scanning with palpation. Several other reports on the risk for malignancy in irradiated thyroid glands, the natural history of benign thyroid nodules, and the prevalence of occult thyroid cancer were also reviewed. These articles formed the basis of our recommendations for treating thyroid incidentalomas. Data Synthesis Reliability of Clinical Examination Because the thyroid gland is located superficially, it is easily palpated. There is disagreement about whether thyroid palpation is best done from the front or the back of the patient and about which system best describes the size of the thyroid gland-one based on estimated weight or one based on other variables, such as the presence of a visible prominence [5]. However, no study has compared the results of different methods of thyroid examination or size determination, which makes it difficult to recommend the use of one method over another [6, 7]. Most nodules that are 1 cm in diameter or larger can be palpated, especially when they are favorably situated. A careful examination should record the size, shape, and consistency of the gland and the number, dimensions, and consistencies of any nodules. A nodule that is located deep within or on the posterior surface of the gland is more difficult to palpate than is one located on the anterior surface [2]. In patients with short, fat necks, nodules may be extremely difficult to detect. Moreover, even with experience and careful technique, physicians may fail to detect many nodules smaller than 1 cm in diameter [8]. In a study by Brander and colleagues [2], one half of the nodules discovered on ultrasonography had escaped detection on clinical examination; approximately one third of the nodules that had not been detected by palpation were larger than 2 cm in diameter. However, a prominent but normal thyroid gland in a patient with a thin neck may be perceived by an examiner as an abnormality of the thyroid [5]. The accuracy of thyroid palpation depends greatly on the experience of the examiner. Interobserver variation in nodule examination has been assessed in two studies. Brander and colleagues [2] discovered a good correlation among examiners in the assessment of thyroid size and classification of nodularity. Most of the examinations were done by internists, but some were done by residents. In contrast, Veith and coworkers [9] found that in one third of cases, examiners disagreed about the number of nodules present. In another study [10], interobserver variation was shown to be less among examiners who had more experience than among those who had different levels of training. We found no study that compared the accuracy of palpation done by thyroidologists or endocrinologists with that of palpation done by general internists. Clinical palpation is thus not a precise tool for assessing abnormality of the thyroid gland, and its reliability is influenced by the size and location of the nodule, the size and shape of the neck, and the experience of the examiner. In one study [11], the sensitivity of palpation of the thyroid gland in terms of size and nodularity was 38%. Autopsy Data In 1955, Mortensen and colleagues [12] examined thyroid glands removed during autopsy from 821 patients at the Mayo Clinic. These glands had all been found to be normal on clinical examination. The authors reported that 406 glands (49.5%) contained one or more nodules; 306 of these (37.3% of 821) were multinodular, and 100 (12.2% of 821) contained single nodules. Of the 406 nodular glands, 144 (35.5%) had nodules that were larger than 2.0 cm in diameter. In an autopsy study of 200 patients with nodular goiter, Hermanson and associates [13] compared the clinical evaluation of thyroid nodularity with the results of pathologic examination in 190 patients. Of 137 patients who had solitary nodules found on clinical examination, 43 (31%) had several nodules found on pathologic examination. In an autopsy series of 215 patients who did not have thyroid disease, Furmanchuk and coworkers [14] documented nodules in the thyroids of 70 patients (32.5%). Thyroid Imaging Studies Current ultrasonographic technology permits high-resolution imaging of the thyroid gland that is more accurate than clinical palpation or other imaging techniques [8, 15, 16]. Ultrasonography is safe and sensitive and is capable of detecting lesions as small as 1 to 3 mm in the thyroid parenchyma [17]. Katz and colleagues [18] reviewed the accuracy of thyroid ultrasonography in 28 thyroid glands examined at autopsy. The correlation between the ultrasonographic finding of thyroid nodules and the pathologic finding of adenomatous goiter was good; ultrasonography thus had a sensitivity of 89% and a specificity of 84%. Tan and associates [8] recently reported that in 151 patients with a clinical diagnosis of a solitary thyroid nodule, ultrasonography showed that 73 (48%) had other nodules (Table 1). Among the patients in whom subclinical nodules were discovered on ultrasonography, 49 (67%) had two nodules and the other 24 (33%) had three or more nodules that had escaped clinical detection. In that report [8], 89% of clinically palpable nodules were 1 cm in diameter or larger. In 72% of patients with more than one nodule, nodules that had not been identified by palpation were smaller than 1 cm in diameter. Table 1. Frequency of Discovery of Additional Thyroid Nodules on Ultrasonography in Patients in Whom Solitary Nodules Were Detected on Palpation In a retrospective analysis, Brander and colleagues [2] compared results of clinical examination with those of ultrasonography and found that only 12 of 32 (38%) clinically solitary nodules were truly solitary on ultrasonographic examination; 15 patients (47%) had several nodules, and 5 had normal glands. As did Tan and associates [8], Brander and colleagues also found that most nonpalpable nodules were smaller than 1 cm in diameter. Walker and coworkers [19] reported that of 200 patients with nodules that appeared to be solitary on clinical examination, 39 (20%) had more than one nodule found on ultrasonography. The sensitivity of thyroid scintigraphy done using technetium Tc 99m pertechnetate was evaluated in the diagnosis of nodular glands. Arnold and colleagues [20] showed that among patients who had clinically normal thyroid glands or equivocal findings, 40% had evidence of one or more lesions on scintigraphy. From these studies, it is clear that thyroid incidentalomas are common in apparently normal glands and in glands with solitary nodules and that they are detected on thyroid ultrasonography, scintigraphy, or both. A nodule smaller than 1 cm in diameter often escapes clinical palpation unless it is located superficially. Prevalence Studies In 1982, Carroll [21] found at least one incidental thyroid nodule in 13% of patients who had carotid ultrasonography (Table 2). In anot
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