Stunning of thyroid tissue by diagnostic activities of (131)I has been described by some investigators and refuted by others. The support both for and against stunning has at times been enthusiastic and vigorous. We present the data from both sides of the debate in an attempt to highlight the strengths and deficiencies in the investigations cited. Clinical, animal, and in vitro studies are included. There are considerable differences in clinical practice, such as the administered activity for diagnostic whole-body scan, delay between diagnostic scan and treatment, time between treatment and posttherapy scanning, and timing of follow-up studies, that have to be analyzed with care. Other factors that often cannot be judged, such as levels of thyroid-stimulating hormone and serum iodine at time of diagnostic testing versus treatment could have an influence on stunning. Larger diagnostic doses and longer delays to therapy appear to increase the likelihood of stunning. The stunning effect of early-absorbed radiation from the therapy should also be considered.
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