**1. Introduction**

A radioiodine whole body scan relies on the fact that differentiated thyroid cancer is more efficient at trapping circulating radioiodine than any other tissues.(Hyer, Newbold et al. 2010) Therefore, when I-131 is administered it accumulates in the thyroid cancer tissues and a radioiodine whole body scan plays an important role in the management of patients with differentiated thyroid cancer. Uptake of iodine by the cancer is related to the expression of sodium iodide symporter (NIS), which actively transports iodide into the cancer cells. Extrathyroidal tissues, such as stomach, salivary glands and breast, are known to have the NIS expression and the organs can physiologically take up iodine.(Riesco-Eizaguirre and Santisteban 2006)

On a whole body scan with diagnostic or therapeutic doses of I-131, except for the physiological radioiodine uptake in the salivary glands, stomach, gastrointestinal and urinary tracts, the lesions with radioiodine uptake can be considered as metastatic lesions in thyroid cancer patients who previously underwent total thyroidectomy.

However, a variety of unusual lesions may cause a false positive result on the radioiodine whole body scan and so careful evaluation of an abnormal scan is imperative to appropriately manage patients with differentiated thyroid cancer.(Mitchell, Pratt et al. 2000; Shapiro, Rufini et al. 2000; Carlisle, Lu et al. 2003; Ahn, Lee et al. 2011) The decision to administer radioiodine treatment is mainly based on the diagnostic scan, and misinterpretation of physiological or other causes of radioiodine uptake as metastatic thyroid cancer could lead to the decision to perform unnecessary surgical removal or to administer a high dose of I-131, which results in fruitless radiation exposure. Therefore, correct interpretation of the diagnostic scan is critical for the proper management.

Physiologic iodine uptake, pathologic iodine uptakes that are not related to thyroid cancer and contamination by physiologic excretion of tracer on the whole body scan are presented and discussed in this chapter. The purpose of this chapter is to make readers consider the possibility of physiologic or pathologic false positive uptake as a reason for the tracer uptake seen on the radioiodine whole body scan.
