Answer to Question #8409 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues

The following question was answered by an expert in the appropriate field:

Q
What is the justification for performing thyroid bioassays on administering staff 24-72 hours after an iodine-131 therapeutic administration? Why not two hours after administration, or 96 hours?
A
A nuclear medicine physician recently told me that it takes anywhere from 24 to 36 hours to obtain the maximum uptake of iodine to the thyroid gland. At four to six hours post administration, the uptake is approximately 6 to 18 percent (Mettler 2006). So most facilities do not perform a bioassay at less than 24 hours.

At 96 hours, decay of the radionuclide plus recycling of the iodine are occurring, which will decrease the accuracy of the potential uptake estimate when trying to back calculate. By then you have lost the opportunity to obtain the biological half-life of the iodine in the individual, if there was an uptake.

Marcia Hartman, MS

Reference
Mettler FA, Guiberteau MJ. Essentials of nuclear medicine imaging; 2006: 77.

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