| Summary: | <i>Objective:</i> To compare brain metabolism using [<sup>18</sup>F]fluorodeoxyglucose ([<sup>18</sup>F]FDG) positron emission tomography/computed tomography (PET/CT) in total thyroidectomy patients during hypothyroidism (levothyroxine withdrawal) or under recombinant human thyroid-stimulating hormone (rhTSH) against levothyroxine intake. <i>Methods:</i> A total of 12 patients were randomly divided into two groups. One group underwent the first [<sup>18</sup>F]FDG PET/CT brain scan after levothyroxine withdrawal (hypothyroidism condition) and repeated the scan 6 months later during regular levothyroxine intake (replacement condition). The other group underwent the first [<sup>18</sup>F]FDG PET/CT scan after receiving an rhTSH injection and maintained regular levothyroxine intake (rhTSH condition), and repeated the scan 7 months later during regular levothyroxine intake without rhTSH administration. The intra-group regional brain metabolisms were compared. <i>Results:</i> Under the hypothyroidism condition, brain metabolism was significantly reduced, namely in the bilateral pre-frontal, temporal, anterior cingulate, and primary motor cortices, insula, and striatum (uncorrected voxelwise <i>p</i> < 0.005); No significant differences were found between the rhTSH and replacement conditions. <i>Conclusion:</i> rhTSH administration could be a better option than levothyroxine withdrawal for <sup>131</sup>I treatment, serum thyroglobulin measurement, or radioiodine scanning for patient follow-up.
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