After reviewing data from Chernobyl regarding estimated doses and cancer risks in exposed children, the FDA provided recommendations for KI supplementation for various populations, depending on their relative risks (Table A43–1).5,16
TABLE A43–1.Threshold Radiation Exposure Doses and Recommended Potassium Iodide KI Doses for Different Risk Groups |Favorite Table|Download (.pdf) TABLE A43–1. Threshold Radiation Exposure Doses and Recommended Potassium Iodide KI Doses for Different Risk Groups
| ||Predicted Thyroid Exposure || || ||Milliliters of Oral Solution, |
| ||Gy ||rad ||KI Dose (mg) ||Number of 130 mg Tablets ||65 mg/mL |
|Adults > 40 years of age ||≥ 5 ||≥ 500 ||130 ||1 ||2 |
|Adults 18–40 years of age ||≥ 0.1 ||≥ 10 ||130 ||1 ||2 |
|Pregnant or lactating women ||≥ 0.05 ||≥ 5 ||130 ||1 ||2 |
|Children and adolescents 3–18 years of agea ||≥ 0.05 ||≥ 5 ||65 ||1/2a ||1 |
|Children 1 month–3 years of age ||≥ 0.05 ||≥ 5 ||32 ||Use KI oral solutionb ||0.5 |
|Children birth to 1 month of age ||≥ 0.05 ||≥ 5 ||16 ||Use KI oral solutionb ||0.25 |
Adults older than 40 years of age face a near-zero risk of developing thyroid cancer from exposure to radioactive iodine. For this group, the complications from iodide supplementation, such as goiter or Graves disease, would likely outweigh any benefit in the setting of relatively mild exposure. However, if public health experts determine that exposure dose may be 5 Gy (500 rad) or greater, which is only likely to occur for those living within a 10 mile radius of a nuclear power plant release, KI is recommended.
Adults 18 to 40 years of age are at risk of developing thyroid cancer that is approximately equal to the risks of side effects of a single dose of iodide supplementation, although it should be understood that both risks are very small. The decision to treat should be based upon the threshold criteria used as well as the risks of iodine supplementation, such as iodine reactions, or a history of past thyroid disease.
Lactating mothers should take KI when instructed to do so by public health authorities. KI is secreted in breast milk and will offer some protection to a neonate, but the risks of treatment are much higher in these patients requiring greater attention to monitoring, as discussed below.
Pregnant women have increased thyroid uptake of iodine, especially in the first trimester, compared with other adults. The developing fetus has increased iodine uptake during the second and third trimesters. Because iodine crosses the placenta, the fetus may potentially be exposed to radioactive iodine, thus supplementation with KI is only recommended at appropriate exposure thresholds.
Children 1 month to 18 years of age are at high risk of thyroid cancer from exposure to radioactive iodine and are at low risk of side effects from supplementation with KI. Therefore, supplementation for children in this age group should begin promptly following official notification of a potential radioactive iodine release.
Neonates are at significantly increased risk from exposure to radioactive iodine because of a marked increase in uptake of iodine resulting from neonatal body cooling in the immediate postdelivery period. At the same time, neonates are susceptible to functional blocking by overloading with stable iodine. Therefore, when supplementation is indicated, KI should promptly be given to neonates with critical attention to dosing. The WHO recommends the KI solution be available for maternity wards in the precise dosing for newborns. Since the most critical time period for thyroid blockage is within the first postpartum week, dosing neonates who are older than one week may be performed at home via dividing, crushing, or suspending tablets in milk, formula, or water.
For full blocking effect, KI should be administered shortly before exposure or as soon as possible afterward. Some models describe the blockade of only 50% of iodine uptake when there is a delay of several hours following an exposure.6 Depending on the duration and type of risk, administration of KI months after an exposure may also partially reduce thyroid cancer risk.6,17
Daily Versus Single Dosing
The protective effect of potassium iodide lasts for about 24 hours, so it should be dosed daily for those groups in whom an ongoing risk is perceived. Depending on the dosing estimates for exposure, this group is primarily children aged 1 month to 18 years. Groups in whom a single dose is recommended include pregnant women and neonates, where there is a significant risk of causing harm to the fetus via impaired cognitive development. For lactating women, stable iodine will be delivered to the nursing newborn and may cause functional blocking of iodine uptake by an overload of iodine. Therefore, the FDA recommends that lactating mothers not receive repeated doses except during continuing, severe contamination, which would generally be defined by health officials.
As mentioned previously, Chernobyl is the most significant massive release of radioiodine in history and nearly all of our experience and recommendations in this type of setting derive from this one event. Although distribution of radioiodine was uneven in Poland, the air concentration of radioiodine had decreased fourfold one week after the first explosion. Therefore, KI prophylaxis was not repeated. It is perhaps based on this experience that it is generally thought that no more than one or two doses of KI would be needed following release of a single radioactive plume, during which other protective measures such as food interdiction or sheltering measures are implemented. While the possibility exists that many repeated releases of radioactive gas might require prolonged prophylaxis, repeat dosing for days to weeks may not be necessary.
Normal thyroid function is critical for proper brain development in a fetus. Just as a fetus and newborn is susceptible to radioiodine uptake, they are also at risk for development of hypothyroidism from repeat dosing of KI. All neonates who are treated with KI in the first weeks of life should be monitored for changes in thyroid stimulating hormone (TSH) and free T4. Likewise when a lactating mother requires repeat doses of KI, the nursing infant should also be monitored for the development of hypothyroidism.