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The So-Called Stunning of Thyroid Tissue

¹ Division of Nuclear Medicine, Department of Radiology, University of Michigan Health System, Ann Arbor, Michigan; and ² Division of Endocrine Surgery, Department of Surgery, University of Michigan Health System, Ann Arbor, Michigan

Correspondence: For correspondence or reprints contact: James C. Sisson, MD, Nuclear Medicine Division, Hospital B1 G505D, University of Michigan Health System, Ann Arbor, MI 48109-0028. E-mail: jsisson{at}umich.edu

When thyroid tissues exhibited concentrations of therapeutic ¹³¹I that appeared to be less than that predicted by data from the preceding diagnostic ¹³¹I, the phenomenon was called stunning. We hypothesized that stunning arose from the early effects of the therapeutic dose of ¹³¹I and that the initial uptake of ¹³¹I, observed within the first day, was not impaired by the diagnostic dose. Methods: The hypothesis was tested by 2 types of studies. In each type, the fractional concentrations of ¹³¹I in residual neck thyroid tissues of patients with papillary thyroid carcinoma were quantified. In the first study, fractional concentrations of diagnostic and therapeutic ¹³¹I were measured at 2 d, a time when stunning has been observed, and expressed as ratios of radioactivity: therapeutic/diagnostic (Rx/Dx). Three different doses of diagnostic ¹³¹I were prescribed to assess a dose response. In the second study, patients were prospectively recruited and tested to record disappearances of radioactivity from thyroid tissues. Diagnostic doses were 1.0 mCi (37 MBq) in all; therapeutic doses were 150 and 30 mCi (5,550 and 1,110 MBq), each to half of the patients. The disappearance curves were extrapolated to the period between 0 and 1 d, an interval when maximum uptake of ingested ¹³¹I would be expected. The fractional concentrations of ¹³¹I at 2 d and at 0–1 d were compared in terms of Rx/Dx ratios to assess changes at each time point. Results: In the first study, after diagnostic doses of 2, 1, and 0.5 mCi (74, 37, and 18.5 MBq), mean 2-d Rx/Dx values in 24, 29, and 17 patients were 0.35, 0.50, and 0.46 (P = 0.087). Of all patients, 74% exhibited Rx/Dx <0.6. In the second study, 6 of 10 patients exhibited disappearance curves of ¹³¹I in which Rx/Dx was <0.6 at 2 d; 5 of the 6 had Rx/Dx values >0.97 at the 0- to 1-d point. In 1 patient the Rx/Dx was 0.54 at 2 d and 0.66 at the earlier time point. The other 4 patients had disappearance curves in which Rx/Dx values were >1.0 throughout or were above 0.6 and did not greatly change. Conclusion: Two days after the administration of ¹³¹I, the mean fractional concentration of radioactivity in thyroid tissues after a therapeutic dose is <60% of the diagnostic dose in most patients, but no correlation of Rx/Dx with the mCi in the diagnostic dose was seen. In 5 of 6 patients in whom the Rx/Dx at 2 d was <0.6, the maximum fractional concentrations of therapeutic and diagnostic ¹³¹I (i.e., the tissue uptakes during the first day) were similar; this pattern was most apparent after therapies with 150 mCi. These results support the hypothesis that "stunning" of thyroid tissues, often observable by 2 d, is primarily the consequence of early destructive effects from therapeutic ¹³¹I.

Key Words: stunning • thyroid carcinoma • radioiodine therapy

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