Thyroid incidentalomas. Prevalence, diagnosis, significance, and management.

During the last 2 decades, the evolution of sensitive diagnostic techniques has significantly improved and simplified endocrine practice. Technologic advances have expanded knowledge and improved management of various endocrine disorders. Unfortunately, technologic advances have also been associated with unintended consequences, leading to serendipitous discovery of abnormalities that may or may not represent disease. For example, the introduction of high-sensitivity assays for thyroid-stimulating hormone (TSH) has resulted in the discovery of slight TSH elevation and suppression recognized as subclinical hypothyroidism and hyperthyroidism, respectively. The significance of these TSH abnormalities, which are associated with normal free thyroid hormone levels, is unknown, and the proper management of these conditions is debated. The increased sensitivity of ultrasonography has led to the detection of nodules in the thyroid gland that are too small to palpate and that have indeterminate clinical significance. According to Black and Welch, 1 "advances in diagnostic imaging create confusion in two crucial areas of medical decision making: establishing how much disease there is and defining how well treatment works." The challenge facing the clinician is to avoid unnecessary diagnostic intervention, treatment, or both that confers little or no benefit. The increased sensitivity of many imaging devices has resulted in the identification of asymptomatic nodules in the thyroid gland. 1,49 The finding of impalpable nodules in an otherwise normal thyroid gland is common simply because thyroid nodules are common. These incidentalomas create a management dilemma for the generalist and the specialist. 49 The discovery of thyroid incidentalomas raises the question of cancer. For microscopic cancer, the reservoir of detectable cancer is huge, particularly in the thyroid gland. The prevalence of clinically apparent thyroid cancer is almost 0.1% in adults 50 to 70 years old. 10 In contrast, Harach and co-workers 18 who dissected the thyroid at 2.5-mm intervals at autopsy reported that the prevalence of histologically proved papillary carcinoma was approximately 100%. When one considers the high frequency of thyroid incidentalomas and the low incidence of clinically apparent thyroid cancer, one conclusion is inescapable, that is, the majority of subclinical cancers never become clinical disease. As Black and Welch 1 write, "despite clinicians' best intentions, many patients may have been labelled with disease they do not really have, and may have been given therapy they do not really need." Thyroid incidentalomas are discovered in several ways. Usually, they are detected in the course of an imaging study for nonthyroid neck disease of neurovascular, arthritic, or parathyroid origin. They also may be detected on ultrasonography as extra nodules in patients with a solitary clinically evident nodule or as part of a screening procedure. Thyroid incidentalomas are discovered during a histopathologic examination in autopsy studies or in surgical specimens of thyroidectomy for nonnodular disease. The optimal management of thyroid incidentalomas has not been clearly established. This article explains how advances in diagnostic imaging have influenced thyroid nodule prevalence. In addition, the diagnosis and clinical significance of benign and malignant thyroid incidentalomas are described. Topics reviewed include the malignant potential of incidentalomas and the concept of "aberrant thyroid cancer." A management strategy is outlined using an evidence-based approach.