Excess free iodide in the blood (ingested or injected) may cause thyrotoxicosis in patients at risk. Iodinated contrast medium solutions contain small amounts of free iodide and may be of significance for patients at risk. The free iodide may also interfere with nuclear medicine diagnostic studies and treatment. Therefore the Contrast Media Safety Committee of the European Society of Urogenital Radiology reviewed the literature on this subject in order to prepare guidelines. A report and guidelines were prepared based on an extensive Medline search. The report was discussed with the participants attending the Tenth European Symposium on Urogenital Radiology, Uppsala, Sweden, September 2003. Contrast medium induced thyrotoxicosis is rare. Contrast medium injection does not affect thyroid function tests (e.g., T3, T4, TSH) in patients with a normal thyroid. Routine monitoring of thyroid function tests before contrast medium injection in patients with a normal thyroid is not indicated even in areas where there is dietary iodine deficiency. Patients at risk of developing thyrotoxicosis after contrast medium injection are patients with Graves' disease and patients with multinodular goiter with thyroid autonomy, especially elderly patients and patients living in areas of iodine deficiency. Patients at high-risk should be carefully monitored by endocrinologists after contrast medium examinations. Prophylaxis in these groups is not generally recommended, although it may offer some protection in selected high-risk individuals. The free iodide load of contrast media injections interferes with iodide uptake in the thyroid and therefore compromises diagnostic thyroid scintigraphy and radio-iodine treatment of thyroid malignancies for 2 months after administration of contrast media. Simple guidelines on the subject are proposed.