Thyroid cancer in children and adolescents has to be considered as the most 
severe health consequence of a nuclear reactor emergency with release of 
radioiodine into the atmosphere. High doses of potassium iodide are effective to 
block radioiodine thyroid uptake and to prevent development of thyroid cancer 
years later. However, there are controversies concerning thyroid cancer risk 
induced by radioiodine exposure in adults. Further, the interaction of 
nutritional supply of potassium iodide and radioiodine uptake as well as the 
interaction of radioiodine with certain drugs has not been addressed properly in 
existing guidelines and recommendations. How to proceed in case of repeated 
release of radioiodine is an open, very important question which came up again 
recently during the Fukushima accident. Lastly, the side effects of iodine 
thyroid blocking and alternatives of this procedure have not been addressed 
systematically up to now in guidelines and recommendations. These questions can 
be answered as follows: in adults, the risk to develop thyroid cancer is 
negligible. In countries, where nutritional iodine deficiency is still an issue, 
the risk to develop thyroid cancer after a nuclear reactor emergency has to be 
considered higher because the thyroid takes up more radioiodine as in the 
replete condition. Similarly, in patients suffering from thyrotoxicosis, 
hypothyroidism or endemic goitre not being adequately treated radioiodine uptake 
is higher than in healthy people. In case of repeated or continued radioiodine 
release, more than one dose of potassium iodide may be necessary and be taken up 
to 1 week. Repeated iodine thyroid blocking obviously is not harmful. Side 
effects of iodine thyroid blocking should not be overestimated; there is little 
evidence for adverse effects in adults. Newborns and babies, however, may be 
more sensitive to side effects. In the rare case of iodine hypersensitivity, 
potassium perchlorate may be applied as an alternative to iodine for thyroid 
blocking.