Answer to Question #7189 Submitted to "Ask the Experts"

Category: Pregnancy and Radiation — Radiation effects to embryo/fetus

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

Q
I have to get a thyroid scan, and I have to take a dose of radioactive iodine. If I am pregnant, what effect will it have on the baby? This is just the uptake scan, NOT the radioactive iodine treatment.
A

Thank you for your question. Without knowing the radionuclide (iodine-123 or iodine-131) and the activity administered, I can't offer any specifics. At my organization, we use iodine-123 (37 MBq) for an uptake scan, and if that is not available we use iodine-131 (3.7 MBq). First, though, the Society of Nuclear Medicine and Molecular Imaging and the National Council on Radiation Protection and Measurements (Report 54) recommend that medical tests that may expose the embryo/fetus to radiation be done only in emergency situations when the testing cannot wait until the pregnancy is over or, in this case, until the results of the pregnancy test come back.

This early in the pregnancy, if you are pregnant, the embryo has no thyroid so thyroid uptake is not an issue. The 37 MBq of iodine-123 or the 3.7 MBq of iodine-131 that we give would expose the very early developing embryo to less than 1 mSv (mSv is a unit of radiation absorbed dose)(Wagner et al. 1997). According to published information, that is not enough radiation dose to cause harmful effects this early in the pregnancy.

Kelly Classic
Certified Medical Health Physicist

Reference
Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations: A guide to medical management 2nd Ed. Madison, WI: Medical Physics Publishing; 1997.

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