TO THE EDITOR:
We read with interest the article by Yeung et al. (1) on therapeutic radioiodine uptake in thyroid remnants after tracer dosimetry. The authors reported decreased uptake in thyroid remnants if 131I-based dosimetry preceded therapy. Unlike other studies (2–5) in which a threshold of 111 MBq 131I was observed before thyroid stunning occurred, the authors reported decreased iodine uptake even with doses as low as 37 MBq 131I.
We congratulate the authors on their excellent study. However, we cannot fully agree with their conclusion that the thyroid stunning was caused by radiation damage from the dosimetric tracer dose. Assuming the effect of thyroid stunning, one would expect a correlation between the amount of radioiodine in remnant tissue (diagnostic dose × diagnostic uptake) and the degree of stunning (ratio of diagnostic uptake to therapeutic uptake). This correlation cannot be found in the data presented in the study (r = −0.08).
With respect to this lack of correlation, we believe instead that there is considerable intratherapeutic stunning rather than dosimetric stunning. Within the first hours after therapeutic radioiodine administration, high radiation doses within thyroid remnant tissue can be expected to have major influence on the subsequent iodine clearance rate in the remnant tissue (iodine subsequently taken up in the stomach and radioiodine coming from the liver after deiodination of radioactive thyroid hormones). This theory would also agree with the previous studies that reported a higher threshold for dosimetric doses.
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REPLY:
As we stated in our article (1), we expected a correlation between the radiation dose to the remnant and the ratio of diagnostic uptake to therapeutic uptake (the stunning effect). However, the radiation dose depends on both the amount of radioiodine in remnant tissue and on the tissue mass, which in most cases is not known. Hence, when the diagnostic uptake is low, the radiation dose may be high if the tissue mass is very low. This phenomenon may explain the lack of correlation between the amount of radioiodine in remnant tissue and the stunning effect.
The possibility of intratherapeutic stunning is a good point that we cannot address with our data. We would need to scan the patient at multiple time points after administration of the therapeutic dose. If the hypothesis of Drs. Diehl and Grünwald were correct, then the 131I clearance from the lesion would be faster. Incidentally, if intratherapeutic stunning occurs, performing dosimetry with 123I would also show a reduction in uptake of 131I from the therapeutic administration.
