Research by Virginia scientists may prompt another look at the doses of potassium iodide designed to protect victims from an accidental or terrorist release of radiation.
In Virginia and elsewhere in the United States, emergency officials have stockpiled potassium iodide tablets for residents near nuclear power plants.
Given within a few hours of exposure, potassium iodide protects the thyroid by flooding it with nonradioactive iodine. Once blocked from the thyroid, the radioactive molecules, which can increase the risk of thyroid cancer, are instead excreted in the urine.
In the new study, scientists from the College of William and Mary and the Thomas Jefferson National Accelerator Facility evaluated the protective effects of four different doses of potassium iodide in mice.
The dose comparable to that given adult humans did not completely block the radioactive iodine from the mouse thyroid, but higher doses did.
"We feel a need to be very cautious about extrapolating these findings to human doses," said physicist Robert Welsh of William and Mary. But, he added, health officials might want to evaluate the protective effects of higher doses.
The Food and Drug Administration issues guidance on doses of potassium iodide in radiation emergencies based on weight and age. The recommended dose for a typical adult human is 130 mg.
W&M senior William Hammond, who conducted the research for his thesis, was to present the team's findings yesterday at an American Physical Society meeting in Tampa, Fla.
Radioactive iodine is used in a wide range of medical diagnosis and treatment in humans, such as treatment of thyroid disorders and some kinds of thyroid cancer.
For certain animal studies, the uptake of radioactive iodine must be very precise to allow for better imaging of the organs. Too much uptake can, for example, make more difficulty in brain imaging, Welsh said.
"When we noted the extent to which the iodine was taken up in the mouse thyroid, we decided to study the effectiveness of blocking" with potassium iodide, Welsh said.
To determine the best dose for blocking the uptake of radioactive iodine from the mouse thyroid, the Virginia team looked at the response to the widely used isotope iodine-125 in 20 mice. They evaluated for doses: one comparable to the human adult dose; and doses three, five and 10 times that comparable to the human adult dose.
They used special gamma-ray cameras to image the movement of radioactive iodine for an hour as it coursed through the anesthetized mice.
Such cameras "allow us to map out the distribution of the isotope throughout the mouse body and to follow it from second to second as it travels through the mouse and is subsequently eliminated," Welsh said.
The mouse dose comparable to that typically given to human adults did not completely block the thyroid from taking up the radioactive iodine, the imaging showed.
All the tested doses prevented some level of radioactive molecules reaching the thyroid. But doses five times and 10 times the comparable human dose were twice as effective as the recommended, comparable human dose, according to W&M biologist Eric Bradley, who participated in the research.
Bradley noted that metabolic rate differences between mice and humans, in addition to other factors, might account for the different responses in blocking the radioactive iodine. Still, he acknowledged that the work may raise questions about the current recommended dose for humans.
"While our findings might motivate [research on human dosages], it would be irresponsible of us to say that this work demonstrates a problem with the recommended human dose," he said.
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Submitted: Monday, April 18, 2005 - 12:00am