The risk of iodine-induced thyrotoxicosis in euthyroid patients receiving iodine-containing contrast agents is known to be low, but data on this risk in patients with latent hyperthyroidism are scarce. We investigated the role of thyroid scintigraphy using Tc-99m preceding the application of iodine-containing contrast material to estimate the risk of iodine-induced thyrotoxicosis in patients with low levels of TSH. In a prospective study on 91 patients, thyroid scintigraphy was performed before coronary angiography (CA). In patients with technetium thyroid uptake (TCTU) less than 1%, CA was done without prophylactic drugs (n = 56). Patients with TCTU greater than 1% were treated either with 900 mg of perchlorate or, depending on the autonomous volume, combined with 20 to 60 mg thiamazole. In the 56 patients with TCTU less than 1%, no case of iodine-induced hyperthyroidism occurred within 4 wk after CA. In the patients who received prophylactic drugs, two cases of mild thyrotoxicosis were observed. Our data suggest that in patients with low levels of TSH, the risk of hyperthyroidism after application of iodine-containing contrast media is negligible if TCTU is less than 1%. In these patients, CA can be performed without administration of prophylactic drugs.