Comparison of 131I thyroid uptake versus 99mTcO4 thyroid uptake

Full text: Introduction: The purpose of this study was to compare the use of iodine and technetium in thyroid uptake scans. The object was to determine whether technetium can be substituted for iodine in determining uptake values. Material and methods: In this retrograde study 70 patients were used. 1. Iodine uptake study: Two methods of Iodine-uptake by the thyroid was used: 123I and 131I. (a) Dose level for 1311 was 20MBq; normal uptake value at 2 hours is 20- 50%. (b) Dose level for 131I was 0,5-1,0 MBq; normal uptake value at 24 hours is 20-45%. Uptake was done by probe or gamma camera. 2. 99mTcO4 uptake study: The total activity of 99mTcO4 in a syringe was measured by gamma camera prior to injection. Following injection of the tracer, an image of the thyroid was registered 20 minutes post injection. Using a computer -generated region of interest around the thyroid image, the percentage of injected activity present in the thyroid 20 minutes post injection was calculated. Results: Twelve patients with a tentative diagnosis of thyroiditis were excluded from the study. In patients with thyroiditis 131I uptake percentages and 99mTcO4 scans varied. Three patients were found to have 99mTcO4 scans and 131I uptake values which did not correlate with each other, or with the clinical findings. Fifty five patients were used for statistical calculation with 20% - 45% uptake of 131I taken as normal, and 2,5-5,5 as normal for 99mTcO4. With iodine 20,37% of the patients were found to be hyperactive and with technetium 25,93%.. With iodine 14,81% of the patients were found to be normal compared to 5,56% with technetium. Sensitivity of iodine study: 87%. Sensitivity of technetium study: 91% . Conclusion: In hyperactive patients the values of iodine and technetium correlated well. However. in euthyroid patients the normal values did not correlate well, the sensitivity of technetium being higher than that of iodine. The results of the study suggest that technetium-uptake scans may be done instead of iodine-uptake studies. To make this conclusion firmer: 1. More cases should be studied. 2. Disease free patients should be studied to formulate normal values for technetium-uptake. (author)