Scientific Paper Review— Facing the Nuclear Threat: Thyroid Blocking Revisited

Hänscheid H, Reiners C, Goulko G, Luster M, Schneider-Ludorff M, Buck AK, and Lassmann M. Facing the nuclear threat: thyroid blocking revisited. Journal of Clinical Endocrinology and Metabolism 2011;96:3511-3516.
 

Two common preparations of Potassium Iodide available in the US

Summary of Paper

Background:
Radioiodine-131 is a major fission product released into the atmosphere after a nuclear accident and results in contamination of water and soil. It may be ingested through food and water that is contaminated with 131-iodide. The most recent accident, of course, was the nuclear reactor damage from the Tsunami in Japan.

Radioidine-131 from the Chernobyl accident resulted in a dramatic increase of  thyroid cancer in children who were exposed to the radiation. To prevent thyroid  cancer from thyroid uptake of 131-iodide, the World Health Organization and the U.S. National Research Council have recommended that the potentially exposed population be given tablets containing 100 mg of iodide as potassium iodide to block the uptake of the radioiodide (1).

Potassium iodide is the same form of iodine used to iodize table salt. Potassium iodide floods the thyroid with iodine, thus preventing radioactive iodine from being absorbed (2). Taking potassium iodide immediately after a nuclear accident appears to lessen the risk of developing thyroid cancer.

Sodium perchlorate is a chemical that causes the thyroid to release any iodine that is stored in the thyroid cells (3). Thus, taking sodium perchlorate after a nuclear accident may cause the thyroid to release any radioactive iodine that the thyroid cells have already taken up.

This study was done to see if taking sodium perchlorate might also lower the chance of thyroid cancer after a nuclear accident. To that end, the effectiveness of sodium perchlorate as a blocking agent was compared with the use of potassium iodide in patients exposed to radioactive iodine.

Methods:
Twenty-seven healthy euthyroid subjects with a mean age of 25 years participated in 48 studies of 123-I kinetics in the thyroid. None of these patients were known to have any thyroid disease.

Each volunteer received a small tracer dose of radioactive iodine (I-123), then different amounts of either potassium iodide or sodium perchlorate. The volunteers then had a radioactive iodine uptake (RAIU) test to see how long the radiation stayed in the thyroid.

Results:
The authors found that 100 mg of either potassium iodide or sodium perchlorate was able to reduce the thyroid absorbed dose of radioactive iodine by almost 90%.

Iodine kinetics were significantly faster in subjects younger than 25 years of age, as compared with those older than 25 years. The time of intervention to achieve a 50% dose reduction was 2.4 hours fort he person with the fastest kinetics and 9.2 hours for the subject with the slowest iodine kinetics.

Conclusions: 
At a time of a nuclear disaster, shielding from radiation by distance, staying indoors, and taking all other measures to avoid exposure to radiation are important. Public authorities must act quickly to distribute iodide tablets to the population who may be exposed in order to prevent thyroid uptake of 131-I that has strong beta radiation, which can induce DNA damage and result in thyroid cancer (1). The data of this paper show that the best blockade occurs when the blocking agent is given before the patient is exposed to radioiodine.

Currently, potassium iodide is the main compound to take after a nuclear accident and is stockpiled in areas that have nuclear power plants in the United States. The results suggest that both potassium iodide and sodium perchlorate lower radioactive iodine levels to the thyroid after a nuclear accident. Both agents are relatively safe to take as a single dose following a nuclear accident.

Since thyroid effects of low levels of perchlorate in the U.S. environment are controversial, however, it is unlikely that it will be made available to the general U.S. public following a nuclear accident. However, it is another compound that can be used in this situation.

Current guidelines for blocking 131- I uptake are adequate for older individuals but probably overestimate the efficacy of blocking in young people, who have faster kinetics. Perchlorate may be used for thyroid blocking as an alternative for individuals with iodine hypersensitivity or those at risk for iodine-induced hyperthyroidism. The conclusion that thyroid iodine kinetics vary among individuals is not novel, but the point that younger people have faster turnover and may need a larger blocking dose given at an earlier time is important.

Implications of the Study (including Protecting our Dogs and Cats)

What about our companion animals? The iodine kinetics for both dogs and cats tends to be more rapid than in human patients, which could suggest that larger doses of potassium iodide or perchlorate might be needed in case of an emergency.

In one study of normal dogs, the blocking action of both potassium iodide and potassium perchlorate was about 90%, similar to the findings of this present study (5). Potassium iodide was chosen for its limited side effects and more universal utilization. In that canine study, results indicated that 25 mg of potassium iodide is the ideal amount to be administered to the dog. This corresponds to approximately 100 mg for a 70 kg human being (i.e., a dose of 1.4 mg/kg).

For more information about use of potassium iodide in dogs and cats, see my related post on "Radiation Toxicity, Potassium Iodide, and Our Pets."

References:

1. National Research Council. Distribution and  administration of potassium iodide in the event of a nuclear incident, Washington, DC: National Academies Press, 2004.
2. Sternthal E, Lipworth L, Stanley B, Abreau C, Fang SL, Braverman LE. Suppression of thyroid radioiodine uptake by various doses of stable iodide. New England Journal of Medicine 1980;303:1083-1088.
3. Greer MA, Goodman G, Pleus RC, Greer SE. Health effects assessment for environmental perchlorate contamination: the dose response for inhibition of thyroidal radioiodine uptake in humans. Environmental Health Perspectives 2002;110:927-937.
4. Ribela MT, Marone MM, Bartolini P. Use of radioiodine urinalysis for effective thyroid blocking in the first few hours post exposure. Health Physics 1999;76:11-16.

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