Imaging of the Thyroid in Benign and Malignant Disease

The thyroid gland was one of the first organs imaged in nuclear medicine, beginning in the 1940s. Thyroid scintigraphy is based on a specific phase or prelude to thyroid hormone synthesis, namely trapping of iodide or iodide analogues (ie, Tc99m pertechnetate), and in the case of radioactive iodine, eventual incorporation into thyroid hormone synthesis within the thyroid follicle. Moreover, thyroid scintigraphy is a reflection of the functional state of the gland, as well as the physiological state of any structure (ie, nodule) within the gland. Scintigraphy, therefore, provides information that anatomical imaging (ie, ultrasound, computed tomography [CT], magnetic resonance imaging) lacks. Thyroid scintigraphy plays an essential role in the management of patients with benign or malignant thyroid disease. In the former, the structure or architecture of the gland is best demonstrated by anatomical or cross-sectional imaging, such as ultrasound, CT, or even magnetic resonance imaging. The role of scintigraphy, however, is to display the functional state of the thyroid gland or that of a clinically palpable nodule within the gland. Such information is most useful in (1) patients with thyrotoxicosis, and (2) those patients whose thyroid nodules would not require tissue sampling if their nodules are hyperfunctioning. In neoplastic thyroid disease, thyroid scintigraphy is often standard of care for postthyroidectomy remnant evaluation and in subsequent thyroid cancer surveillance. Planar radioiodine imaging, in the form of the whole-body scan (WBS) and posttherapy scan (PTS), is a fundamental tool in differentiated thyroid cancer management. Continued controversy remains over the utility of WBS in a variety of patient risk groups and clinical scenarios. Proponents on both sides of the arguments compare WBS with PTS, thyroglobulin, and other imaging modalities with differing results. The paucity of large, randomized, prospective studies results in dependence on consensus expert opinion and retrospective analysis with inherent bias. With a growing trend not to ablate low-risk patients, so that a PTS cannot be performed, some thyroid carcinoma patients may never have radioiodine imaging. In routine clinical practice, however, imaging plays a critical role in patient management both before and after treatment. Moreover, as evidenced by the robust flow of publications concerning WBS and PTS, planar imaging of thyroid carcinoma remains a topic of great interest in this modern age of rapidly advancing cross sectional and hybrid imaging with single-photon emission computed tomography, single-photon emission computed tomography/CT, and positron emission tomography/CT. Semin Nucl Med 42:49-61 (C) 2012 Published by Elsevier Inc.