Comparison of Tc-99m-methoxyisobutyl isonitrile and Tl-201 scintigraphy invisualization of suppressed thyroid tissue

Both (TI)-T-201 and Tc-99m-methoxyisobutyl isonitrile (MIBI) have been used in the visualization of suppressed thyroid tissue in patients with autonom ously functioning thyroid nodules (AFTNs). It has been suggested that thyro id-stimulating hormone (TSH) control is not a major determinant of both tra cers. However, the mechanism of thyroid uptake of these agents is controver sial. In this study, we compared (TI)-T-201 and MIBI in the visualization o f suppressed thyroid tissue in patients with a solitary toxic AFTN. Methods : Thirty-two patients (13 triiodothyronine [T-3] and 19 T-3 + levorotatory thyroxine [T-4] hyperthyroid patients) with toxic AFTNs Visualized on Tc-99 m-pertechnetate scanning were included in the study. All patients underwent MIBI and (TI)-T-201 thyroid scintigraphy within a 3-d interval. The scinti grams were analyzed both visually and semiquantitatively. For the semiquant itative analysis, regions of interest (ROIs) were generated over the nodule (N) and contralateral normal lobe (E), and the mean counts in each ROI wer e calculated. Results: The N/E uptakes (mean +/- SD) for pertechnetate, MIB I, and (TI)-T-201 were 11.37 +/- 4.53, 4.76 +/- 1.33, and 1.63 +/- 0.15, re spectively, in T-3 +/- T-4 hyperthyroid patients and 9.46 +/- 3.64, 2.73 +/ - 0.63, and 1.57 +/- 0.23, respectively, in T-3 hyperthyroid patients. Our results showed that (TI)-T-201 uptake of suppressed thyroid tissue compared with AFTN was more prominent and significantly higher than that of MIBI fo r both groups of patients (P = 1.08E-05 for T-3 and 6.15E-09 for T-3 + T-4 hyperthyroidism). There was no significant difference for either pertechnet ate or (TI)-T-201 (P > 0.05) when the N/E uptakes of both groups of patient s were compared. However, the N/E uptake of MIBI in T-3 + T-4 hyperthyroid patients was significantly higher than that in T-3 hyperthyroid patients (P = 6.69E-06). Conclusion: Clear Visualization of suppressed thyroid tissue with both (TI)-T-201 and MIBI in patients with low serum concentrations of TSH suggests that TSH is not a major factor in the thyroid uptake of either agent. (TI)-T-201 is superior to MIBI in the visualization of suppressed t hyroid tissue in patients with a toxic thyroid nodule. An increased rate of metabolism in the follicular cells of AFTNs in T-3 + T-4 hyperthyroid pati ents compared with that in T-3 hyperthyroid patients might be responsible f or the higher N/E for MIBI compared with that for (TI)-T-201.