The purpose of this study was to asses the detectability of differentiated thyroid carcinoma (DTC) metastases by 99mTc-tetrofosmin and to compare the results of 99mTc-tetrofosmin with 131I and 201Tl. The reliability of 201Tl and 99mTc-tetrofosmin scanning during suppression 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 201Tl, 99mTc-tetrofosmin or 131I 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 201Tl and 99mTc-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 201Tl, 99mTc-tetrofosmin and 131I in diagnosing distant metastases were comparable (0.85, 0.85 and 0.78, respectively). Iodine-131 was much more sensitive than 201Tl and 99mTc-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. CONCLUSIONS Technetium-99m-tetrofosmin and 201Tl imaging are highly sensitive for detecting differentiated thyroid carcinoma metastases and do not require prior withdrawal of thyroid hormone suppressive therapy.