Efficacy of Thyroid Blockade on Thyroid Radioiodine Uptake in 123I-mIBG Imaging

Although iodinated radiopharmaceuticals usually contain a small quantity of unbound iodine, it is difficult to establish the degree to which thyroid activity on scintigraphic images reflects uptake of free radioiodine. The objective of the present study was to examine the effectiveness of thyroid blockade in subjects undergoing I-123-meta-iodobenzylguanidine (mIBG) imaging and to estimate the relative contribution of bound and unbound radioiodine to imaging findings. Methods: All subjects were participants in prospective trials of I-123-mIBG cardiac imaging in which pretreatment with thyroid blockade was optional unless locally required. In a pilot project, 15 subjects (6 blocked) had thyroid uptake measured at 4 h using a probe system. Fifteen-minute (early) and 4-h (late) anterior planar chest images that included the thyroid region were visually scored for thyroid uptake (scale of 0-4) in another group of 152 subjects (98 blocked). Quantitative analysis based on thyroid regions of interest was performed on anterior planar images from a further sample of 669 subjects (442 blocked). For all 3 investigations, quantitative comparisons of thyroid uptake were made between the blocked and nonblocked subjects. Results: There was no statistical difference between probe uptake of the 6 blocked and 9 nonblocked subjects. However, in the second series, mean visual score on the late images was significantly lower for blocked than nonblocked subjects (P < 0.001). In the region-of-interest analyses, net thyroid counts were significantly higher on the late images of nonblocked subjects (P < 0.0001), and compared with early images, 87% of subjects who received blockade showed decreased or unchanged counts whereas 75% of nonblocked subjects had increased net thyroid activity. In nonblocked subjects, an estimated 79% of thyroid counts on late images could be attributed to unbound I-123. Conclusion: On the basis of 3 different methods for assessing thyroid uptake of I-123, use of thyroid blockade pretreatment in I-123-mIBG imaging prevents increase of thyroid activity over time because of uptake of unbound I-123. In most subjects, there is a low level of I-123-mIBG thyroid activity that probably represents specific uptake in sympathetic nerve terminals.