The purpose of this study was to asses the detectability of differentiated thyroid carcinoma (DTC) metastases by Tc-99m-tetrofosmin and to compare the results of Tc-99m-tetrofosmin with I-131 and Tl-201. The reliability of Tl-201 and Tc-99m-tetrofosmin scanning during supression therapy also has been studied. Methods: A prospective study was performed on 41 patients (30 females, 11 males) with DTC (30 papillary, 11 follicular) who had undergone total thyroidectomy and received an average dose of 117 mCi (4329 MBq) of radioiodine for ablation of postsurgical residual thyroid tissue. All patients (n = 41) had Tl-201, Tc-99m-tetrofosmin or I-131 whole-body imaging after discontinuation of thyroid hormone replacement (thyroxine-off group), Eight of 14 patients with distant metastases also were imaged when they were on thyroxine therapy both with Tl-201 and Tc-99m-tetrofosmin (thyroxine on-and-off group). Radiologic studies (chest radiography, CT and MRI), serum thyroglobulin assays and histopathologic examinations were performed to clarify the presence of metastases with positive uptake on any of three radionuclide studies. Results: In 26 of 41 patients all three scans were negative. These patients also clinically didn't show any evidence of metastases, Fourteen patients were considered to have distant metastases on the basis of clinical, radiologic and histopathologic findings. The sensitivities of Tl-201, Tc-99m-tetrofosmin and I-131 in diagnosing distant metastases were comparable (0.85, 0.85 and 0.78, respectively). iodine-131 was much more sensitive than Tl-201 and Tc-99m-tetrofosmin for demonstrating residual thyroid tissue after surgery (1.00, 0.33 and 0.33, respectively). The only false-positive case involved radioiodine uptake in a tuberculoma, Thyroxine-on images of 8 patients with distant metastases showed no difference from their thyroxine-off images regarding the site, number and uptake of metastases, Conclusion: Technetium-99m-tetrofosmin and Tl-201 imaging are highly sensitive for detecting differentiated thyroid carcinoma metastases and do not require prior withdrawal of thyroid hormone suppressive therapy.