[ RadSafe ] Protecting the thyroid with KI may require 5X FDA recommended dosage

[John Jacobus]

Received this through another list server, but am
providing the original linkage, which is the Society
of Nuclar Medicine.

http://interactive.snm.org/index.cfm?PageID=3912&RPID=10

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JLab, College of W&M Researchers Study Potassium
Iodide for Radiation Protection in Imaging Studies on
Mice
Posted April 21, 2005 
Source: DOE/Thomas Jefferson National Accelerator
Facility

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Scientists have found that a dose five times higher
than the FDA-recommended dosage of potassium iodide
for protection of humans in the event of a nuclear
accident is needed to protect small animals
effectively from radioactive iodide in imaging
procedures.

The long-term animal nuclear imaging project is being
conducted by a collaboration of biology and physics
researchers from the Department of Energy's Jefferson
Lab and The College of William & Mary (CWM).

The research was conducted at the CWM with a Jefferson
Lab and CWM-built medical imaging system to perform
investigational studies of mice. Bob Welsh, a JLab/CWM
jointly appointed professor, is one researcher working
on the project. The research demonstrates that
scientists can learn about how the body uses certain
substances of interest—such as insulin, the
fat-regulating protein leptin, and a wide range of
other biological compounds—by tracking how these
substances move through the body of a mouse.

"The way we follow those substances is to attach to
them a radioactive isotope of iodine, Iodine-125.
Iodine-125 emits a low-energy gamma ray," Welsh says,
"It's not a tremendous amount of energy, but it's easy
to track with these very precise detectors that have
been designed and built by the Jefferson Lab Detector
Group."

The thyroid needs iodine to regulate metabolism and is
unable to distinguish between regular dietary iodine
and ingested radioactive iodine. So the researchers
were not surprised when, in the course of the project,
they noticed that the mice subjects' thyroids always
absorbed a significant amount of radioactive iodine.
In addition to being potentially bad for the mouse,
the thyroid's absorption of radioactive iodine made
the images difficult to interpret and could provide
false-positive readings or possibly obscure
substantial iodine uptake in nearby tissues.

The team decided to test what would happen if they
gave the mice potassium iodide, the FDA-recommended
drug for blocking radioactive iodine absorption by the
thyroid in the event of a nuclear accident, before
exposing the mice to a form of radioiodine used in
imaging studies. CWM undergraduate William Hammond,
who will be presenting the team's findings at the
American Physical Society (APS) April Meeting, Session
E12.0004, participated in this phase of the research
for his senior thesis project.

The researchers started with the potassium iodide dose
that is recommended for humans in the event of a
nuclear incident, 130 mg, and scaled that down
proportionally to the mass of the mouse. They
administered a liquid form of the drug to mice,
injected the radioiodine for imaging an hour later,
and imaged the mouse.

"What we noticed was this: the dose that was exactly
the scaled human dose did not completely block the
uptake of radioiodine. But when we tried three times,
five times, ten times the scaled human dose, we
obtained results that indicate that ten times the
human dose blocks 1.5-2 times better, though five
times is just about as good as ten times," Welsh says.

The researchers recognized that the extra benefit
gained by the largest potassium iodide dose
administered could in some cases be outweighed by
potential side effects. To protect their mice in
future imaging studies, they're planning to use the
potassium iodide dose that is five times the
scaled-down human dose.

As for larger implications, the study should not
automatically be applied to humans. "It could say that
a mouse's metabolism is so different from a human's
that you can't just scale the human dose down for
mice. But when it comes to small animals, I think the
results should be taken into consideration," Welsh
notes.

This research was made possible by a collaboration of
Jefferson Lab and College of William researchers,
including CWM physicists Robert Welsh, Julie Cella,
Coleen McLoughlin, Kevin Smith and William Hammond;
CWM biologists Eric Bradley and Margaret Saha; CWM
applied science graduate student Jianguo Qian; and
Jefferson Lab Detector Group scientists Stan Majewski,
Vladimir Popov, Mark Smith and Drew Weisenberger.


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"Embarrassed, obscure and feeble sentences are generally, if not always, the result of embarrassed, obscure and feeble thought."
Hugh Blair, 1783

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com

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