Thyroid stunning

Debates regarding thyroid stunning-a phenomenon whereby a diagnostic dose of radioiodine decreases uptake of a subsequent therapeutic dose by remnant thyroid tissue or by functioning metastases-have been fueled by inconsistent research findings. Quantitative studies evaluating radioiodine uptake and qualitative studies using visual observations both compare thyroid function on the diagnostic scan (DxSCAN) versus the posttreatment whole-body scan (RxWBS). The variability of findings may be the result of a lack of consensus in clinical nuclear medicine regarding many parameters of radioiodine usage including the need to obtain a pretreatment diagnostic scan, appropriate therapeutic dose, time between therapy dose administration and DxSCAN, and how successful ablation is measured. In the studies considered in this review, those that used I-123 rather than I-131 for DxSCAN, allowed less time to elapse between diagnostic and therapy dose, and more time between therapy dose and RxWBS (at least I week), did not observe stunning. However, groups that recognized stunning did not demonstrate any difference in outcomes (determined by successful first-time ablation). Whether stunning is a temporary phenomenon whereby stunned tissue eventually rejuvenates, or whether observed stunning actually constitutes partial ablation, is yet to be delineated.