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2/7/2002

FDA Offers Guidance on Prophylaxis for Exposure to Radioiodines

Kate Traynor

A guidance document released last December by FDA discusses the use of oral potassium iodide as a blocking agent to protect against exposure to radioactive iodine in the event of a nuclear power plant disaster.1

The rationale behind potassium iodide prophylaxis is to prevent cancer-causing radioisotopes of iodine from binding to the thyroid. FDA noted in its guidance document that potassium iodide prophylaxis protects just one organ—the thyroid—against inhaled and ingested radioiodines. The treatment, FDA said, "has no impact on the uptake by the body of other radioactive materials and provides no protection against external irradiation of any kind."

Radioactive isotopes of potassium iodide are normal fission products found in the fuel rods of nuclear reactors. An explosion in a reactor’s energy-producing core, such as the one that occurred during the 1986 Chernobyl nuclear reactor accident in the Ukraine, can release radioactive iodine into the atmosphere.

According to the guidance document, FDA relied heavily on data obtained after the Chernobyl accident to formulate the agency’s potassium iodide prophylaxis recommendations. The agency emphasized that potassium iodide prophylaxis should be an adjunct to evacuation, control of potentially contaminated food sources, and other ways of minimizing exposure to radiation.

FDA advised that potassium iodide prophylaxis is most effective when started "before or immediately coincident with passage of the radioactive cloud." The treatment also offers some protection when begun three to four hours after exposure to radioiodines.

Potassium iodide dosage recommendations made by FDA depend on the age of the victim and the amount of radioactive iodine to which the person is exposed. Exposure to radioiodines is expressed in centigrays (cGy), a measure of the absorbed radiation dose.

After exposure to a low level of radioiodines, the risk of developing thyroid cancer is high in young children, according to FDA’s analysis. The agency set 5 cGy as the minimum exposure level at which all infants, children, and pregnant or lactating women should begin potassium iodide prophylaxis.

At this exposure level, newborns up to one month of age should receive potassium iodide 16 mg/day until they are no longer at risk for inhaling or ingesting radioiodines. During treatment, neonates should be monitored for signs of hypothyroidism and given thyroid hormone therapy if necessary.

At an exposure level of >5 cGy, infants and children from one month through three years of age should receive potassium iodide 32 mg/day. Children older than three years and most adolescents through 18 years of age should receive potassium iodide 65 mg/day. Pregnant or lactating women and adolescents who weigh at least 70 kg should receive the adult dosage—potassium iodide 130 mg/day.

FDA said that 65- and 130-mg potassium iodide tablets can be split into smaller doses, if necessary. A freshly prepared saturated potassium iodide solution can be diluted with milk, formula, or water for administration to infants and children.

FDA recommends that adults up through 40 years old start potassium iodide prophylaxis at 130 mg/day if their predicted exposure level is >10 cGy. For adults over 40 years, prophylaxis is recommended only if the predicted exposure level is >500 cGy; the dosage for this group is also 130 mg/day.

FDA emphasized that "across populations at risk for radioiodine exposure, the overall benefits of KI [potassium iodide] far exceed the risks of overdosing." The guidance document noted that iodine-induced thyrotoxicosis, iodide goiter, and hypothyroidism may occur after repeated use of potassium iodide. These adverse events are much more likely to occur in adults than in children.

In an announcement made last December, the Nuclear Regulatory Commission (NRC) said that it has set aside $800,000 to stockpile potassium iodide and make it available to states on a "first-come, first-served basis."

"We decided that we would fund the initial supplies," said NRC Public Affairs Officer Rosetta Virgilio. She described the stockpile as providing "one or two pills per person within the 10-mile emergency planning zone" surrounding each nuclear power plant in the United States.

NRC’s offer extended beyond the geographic limits of the states that have nuclear power plants. "Thirty-two states have nuclear power plants," Virgilio said. "But possibly 34 could need KI, because the state of Delaware is near a couple of plants in New Jersey," she said. Areas of West Virginia are similarly within the 10-mile zone of a power plant in Pennsylvania.

So far, states’ interest in NRC’s offer to stockpile potassium iodide has been low. "We really aren’t seeing the states breaking down the doors to get this," Virgilio said. By January 11, just two states—Massachusetts and Maryland—had formally notified NRC that they wanted to participate in the program.

Virgilio said that emergency-preparedness programs in Arizona, Alabama, and Tennessee already make potassium iodide available to people who live near nuclear power plants in those states.

  1. Center for Drug Evaluation and Research. Guidance: potassium iodide as a thyroid blocking agent in radiation emergencies. Rockville, MD: U.S. Department of Health and Human Services; 2001.