Both (TI)-T-201 and Tc-99m-methoxyisobutyl isonitrile (MIBI) have been used in the visualization of suppressed thyroid tissue in patients with autonomously functioning thyroid nodules (AFTNs). It has been suggested that thyroid-stimulating hormone (TSH) control is not a major determinant of both tracers. However, the mechanism of thyroid uptake of these agents is controversial. In this study, we compared (TI)-T-201 and MIBI in the visualization of 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-99m-pertechnetate scanning were included in the study. All patients underwent MIBI and (TI)-T-201 thyroid scintigraphy within a 3-d interval. The scintigrams were analyzed both visually and semiquantitatively. For the semiquantitative analysis, regions of interest (ROIs) were generated over the nodule (N) and contralateral normal lobe (E), and the mean counts in each ROI were calculated. Results: The N/E uptakes (mean +/- SD) for pertechnetate, MIBI, and (TI)-T-201 were 11.37 +/- 4.53, 4.76 +/- 1.33, and 1.63 +/- 0.15, respectively, 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 for 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 pertechnetate or (TI)-T-201 (P > 0.05) when the N/E uptakes of both groups of patients 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 thyroid 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 patients compared with that in T-3 hyperthyroid patients might be responsible for the higher N/E for MIBI compared with that for (TI)-T-201.