Guidelines for Iodine Prophylaxis following Nuclear Accidents

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Guidelines for iodine prophylaxis following nuclear accidents1999 updatewill be influenced by breathing rates and intake through ingestion willbe influenced by dietary habits.In the following, the risks from radiation exposure and the risks fromstable iodine prophylaxis, respectively, are examined in more detailfor the various population groups.Pregnant womenDuring pregnancy, the maternal thyroid gland is stimulated, especiallyduring the first trimester. The fraction of radioactive iodine taken upby the thyroid is increased as compared to other adults. Thus, there isa greater need to protect the thyroid gland of the pregnant woman.During the second and third trimesters, the thyroid gland of thedeveloping fetus takes up and stores iodine in increasing amounts.Iodine passes readily across the placenta, and thus, after the firsttrimester, the fetal thyroid gland can be exposed to radioactive iodinethrough the placenta, but it can also be protected by stable iodinetaken by the mother. However, the risk of blocking the fetal thyroidfunction by a prolonged overload of stable iodine must be kept inmind, especially in areas with inherent dietary iodine deficiency.While there are physiological differences between the trimesters,outlined above, there is no need for a different policy of intervention,which would create substantial problems in practice. Throughoutpregnancy, the number of stable iodine doses should be kept to theminimum needed to provide adequate protection against inhaledradioactive iodine. No negative consequences are to be expected afterone or two doses of stable iodine. However, especially in areas withdietary iodine deficiency, prolonged dosage could lead to maternaland/or fetal thyroid blockage, with possible consequences for fetaldevelopment. It is important, therefore, that this be avoided. Toprotect against ingestion of radioactive iodine, which would implyrepeated doses of stable iodine, appropriate food control measuressuch as the provision of uncontaminated milk must be given priority.If stable iodine is given late in pregnancy, there is a need to monitorthe newborn for thyroid function, but this would be met by routinescreening programmes already in place in most countries. Pregnantwomen with active hyperthyroidism must not take stable iodinebecause of the risk of fetal thyroid blockage.9
Guidelines for iodine prophylaxis following nuclear accidents1999 updateNeonatesNewborn infants are quite likely the critical group of concern whendeciding on the implementation of stable iodine prophylaxis. In thefirst few days of life they are at special risk both of exposure fromradioactive iodine and blocking of thyroid function by an overload ofstable iodine.After birth, there is a dramatic increase in thyroid activity, lasting onlya couple of days. The fraction of radioactive iodine intake that will beincorporated into the thyroid at this critical stage can be fourfoldgreater than for all other age groups (15). On the other hand, duringthis period the thyroid is especially sensitive to the functional blockingcaused by an overload of stable iodine. The most critical period fordeveloping thyroid blockage lasts for less than a week, even in thepremature. Even transient hypothyroidism during the critical period ofbrain development can result in loss of intellectual capacity (16). Thepotential for harmful influence on neurointellectual development,however, was not confirmed in the Polish study referred to insection 2.3.When indicated, stable iodine in the form of potassium iodide (KI)will be promptly given to all neonates. The dosage is critical. A singleadministration of 12.5 mg iodine (16 mg KI) should not be exceeded.If stable iodine is given, close follow-up is essential.KI solution should be readily available in maternity hospitals. Thiswill enable prompt and exact dosage to the critical group of thenewborn still on the ward. A few days later the sensitivity forblockage of thyroid function will have decreased and dosage may beperformed at home, by dividing, crushing and suspending tablets inmilk or water. In infants who have been administered stable iodine inthe first weeks of life, TSH levels and, if indicated, T 4 levels will bemonitored and appropriate replacement therapy given.Infants, children and adolescents (1 month to 18 years)These groups are at high risk from exposure to radioactive iodine butat very low risk from stable iodine. The dose to the thyroid fromradioactive iodine in a given situation will be higher in this group thanin adults because of the smaller size of the gland, which is only partlycompensated for by a smaller breathing volume. The highest dose10
Page 1 and 2: WHO/SDE/PHE/99.6English onlyDist.:
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Guidelines for Iodine Prophylaxis following Nuclear Accidents

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