Simple, rapid thyroid function testing with 99mTc-pertechnetate thyroid uptake ratio and neck/thigh ratio

To avoid the technical difficulties and errors inherent in the measurement of early thyroid uptake of 99mTcO4-,techniques which are independent of absolute uptake, neck extrathyroidal background and dose standards were evaluated in a series of 108 patients. After intravenous injection of 2 mCi 99mTcO4-, radioactivity was recorded over the neck and thigh. Thyroid uptake ratios were calculated as the ratios of activity over the neck at two times. A neck/thigh ratio was calculated from the recorded activities at 15 min after injection. Examination of these parameters showed that a combination of the 15 min neck/thigh ratio and the 10'/2' thyroid uptake ratio best served to discriminate thyroid function: 92% of hyperthyroid cases were correctly identified by a neck/thigh ratio above 4.7 and 95% of hypothyroid cases were identified by the combination of a neck/thigh ratio below 3 and a 10'/2' thyroid uptake ratio below 1. Correct classification of euthyroidism was 84% but with the exclusion of patients previously treated with 131I, this rose to 91%. The accuracy of the 99mTc procedure is comparable to that of the standard 24 hr 131I uptake run concurrently in this series and duplicates the accuracy of computer assisted determinations of absolute thyroid 99mTcO4- uptakes. The procedure provides a convenient method for the evaluation of thyroid function as an accompaniment to 99mTcO4- thyroid imaging.
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