Some Comparative Studies Using ^99mTc and ¹³¹I in Thyroid Scanning

Summary

By comparing thyroid scans made with ^99mTc and ¹³¹I in 40 euthyroid goiter patients large discrepancies were found in 20% of all cases. For example, “warm” nodules on ^99mTc scans were found to be “cold” on ¹³¹I scans, or a “cold” nodule on a ^99mTc scan was observed as a “warm” one on a ¹³¹I scan. The above discrepancies seem to result from the differences in the metabolism of the two radioisotopes in the thyroid and those in the time which elapsed between the administration of the radioisotope and the execution of the scan.

Generally speaking ^99mTc thyroid scintigraphy should not be recommended as a routine technique in adults, its usefulness being limited to the cases in which ¹³¹I scans were found to be illegible. “Cold” nodules on ^99mTc scans should also be reexamined after T₃ stimulation.

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