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Thyroid Uptake and Scan
Background
Radioactive iodine (RAI) uptake and scan is a direct, noninvasive test used to assess the function of the thyroid gland. RAI uptake and scan is frequently interchangeable with “RAI scan” or “RAI uptake” in clinical practice. However, the RAI test involves two separate phases including the uptake and the scan. Thus, to be clear, RAI uptake and scan will be the term used in this section. Physiologically, the thyroid gland actively transports and traps iodine inside the cell to synthesize thyroid hormones. The test uses a radioactive isotope of iodine ( 123 I) as a tag for the body’s stable form of iodine (nonradioactive 127 I) to measure the fractional uptake by the thyroid gland. The goal of a RAI uptake and scan is to determine the percent of the radiopharmaceutical that becomes trapped in the thyroid gland. The scan (or scintigraphy) provides an image, allowing for assessment of homogeneity or heterogeneity of the radiotracer distribution within the thyroid gland.
How to Use It
There are several factors that influence the 24-hour thyroid iodide uptake as listed in Box 55.1 . , , RAI uptake and scan is commonly performed to determine the etiology of thyrotoxicosis when the diagnosis is not clear, usually in the case of a patient with nodular thyroid disease. The etiology of thyrotoxicosis includes Graves’ disease, toxic multinodular goiter (MNG), toxic adenoma, thyroiditis, intrathoracic mass, ectopic thyroid tissue, and human chorionic gonadotropin (hCG)-mediated hyperthyroidism. When 123 I is administered to a patient with a normal thyroid gland, the tracer activity will be fairly homogeneous throughout the gland, as shown in Fig. 55.1 . In Graves’ disease, the tracer activity is also homogenous but more intense throughout the gland, as shown in Fig. 55.2 . In cases of MNG ( Fig. 55.3 ), there can be thyroid areas with lower than normal uptake of 123 I that correspond to hypofunctioning thyroid nodules (also called cold nodules), areas with supranormal uptake suggestive of hyperfunctioning thyroid nodules (also called hot nodules), and areas of homogeneous uptake that correspond to thyroid tissue without nodules. A toxic nodule has significant uptake, whereas the remainder of the thyroid uptake is low to absent. On the contrary, in cases of thyroiditis ( Fig. 55.4 ), the tracer activity in the gland will be minimal to absent, reflecting the reduced function of the inflamed thyroid gland. A low thyroid uptake will also be noted in cases of iatrogenic hyperthyroidism, iodine-induced hyperthyroidism, amiodarone-induced thyroiditis, and, rarely, in cases of struma ovarii.
Box 55.1
Factors That Influence 24-Hour Thyroid Iodide Uptake
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