Answer to Question #12109 Submitted to "Ask the Experts"

Category: Pregnancy and Radiation

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


My wife took about 150 megabecquerels (MBq) of iodine-131 (131I) for a whole-body scan to check her thyroid cancer that occurred seven years ago. Two months from the day she took the 131I pill, she missed her period and was pregnant. Will there be any harm to the baby?


There won't be any harm to the baby. There are a couple of reasons why I say this.

First, since your wife conceived and now has a viable pregnancy, if any radioactivity remained in her body, it was too low to cause harmful effects. Would there be any radiation dose to the embryo? Possibly, if conception occurred within six months after the 131I administration, and in this case, it did. Is it a big radiation dose? No.

If there was any radioactivity remaining in your wife's body, the radiation dose would be small—smaller than doses we know to cause effects. We know that doses near 100 millisieverts (mSv, a unit of effective radiation dose) can cause a spontaneous abortion within the first 10 days after conception. The conceptus' radiation dose from your wife's 131I administration would be about equal to or less than 1.0 mSv (Wagner et al. 1997). (Just for comparison, the average radiation dose each of us receives from background radiation is about 3 mSv each year.)

Another thing to consider is the thyroid gland of the conceptus since 131I is taken up by thyroid tissue. But the conceptus has no thyroid. The thyroid gland would not be formed until about weeks 9–12 so there need not be any worry about that.

With a viable pregnancy today, it is unlikely your wife's 131I administration would have caused any harm to the baby.

Kelly Classic
Certified Medical Health Physicist

Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations. Madison, WI: Medical Physics Publishing; 1997.

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 24 August 2017. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.