Answer to Question #958 Submitted to "Ask the Experts"

Q

I was hyperthyroid and in early December 2000 I underwent radioactive iodine therapy. I'm not sure how many rads; I think 15. I took it in pill form and it did work to kill part of my thyroid. I am now six weeks pregnant and wondering what, if any, effects there could be on my unborn baby. It has been over four months since this treatment and my thyroid is fine.

A

I understand there has been some clarification that your pill had 15 mCi of ¹³¹I in it—not that you received 15 rad. If that is correct and because you had the iodine therapy well before you became pregnant there is no need for concern. Let me explain how I arrived at that conclusion. After administration of ¹³¹I, on the average, 80% is eliminated through the urine in the first 48 hours. So, 48 hours after your administration, you had only about 3 mCi left. I'm not sure if you are aware, but radionuclides actually lose their radioactivity over time. The rate of loss for each radionuclide is different. We call this half-life, meaning that after a certain known period of time, there is a known rate of loss. In this case, ¹³¹I has a half-life of 8 days. That means every eight days half of the ¹³¹I radioactivity is gone. Since you originally had 15 mCi at the beginning of December and then a couple of days later had only about 3 mCi, we'll start there. If we assume that all of the rest of the ¹³¹I stayed in your body this whole time, from the beginning of December until the beginning of May (approximate time you became pregnant), the ¹³¹I went through 18 half-lives! That's a lot. That means, conservatively, that only about 0.00038% could even have been left. Even if it was, there would be no measurable dose to the baby with that small amount. I hope this makes some sense. The short answer is that everything is all right.

Kelly Classic
Certified Medical Health Physicist