Answer to Question #3540 Submitted to "Ask the Experts"

In your "Experts' Answers" section there are many questions regarding ¹³¹I but not ¹²³I. I had a thyroid scan done recently with appoximately 350 microcuries of ¹²³I NaI in a capsule form. I would like to become pregnant as soon as possible and was concerned about any residual effects of the radiation. How long should I wait before becoming pregnant to be safe? Also, I have two small children (ages 18 months and three years). Did the amount of radiation I took have any harmful effects on them? I was told to stop breast-feeding for seven days following the scan (I weaned permanently) but no information was given about becoming pregnant.

Iodine-131 (¹³¹I) was the first radioactive isotope of iodine available for medical use. Thus, there is more data applicable to it than to any other iodine isotope. It is still the choice for therapeutic administration, but has been largely replaced for diagnostic scans by ¹²³I which delivers a much smaller dose to the thyroid and to the whole body than its predecessor, largely because of its shorter physical half-life (13 hours for ¹²³I versus eight days for ¹³¹I). Thus it is eliminated from the body much more rapidly. Radiation dose to the patient varies strongly with thyroid activity, or the ability of the thyroid to concentrate the administered iodine. For example, the radiation dose to the thyroid from a standard scan (400 microcuries) using ¹²³I varies from about 1 milligray (for 0% uptake to the thyroid) to 100 (for 55% uptake) milligray. For the same administered dose of ¹³¹I, thyroid dose may be as much as 100 times greater. Whole-body effective dose for ¹³¹I is also about 100 times greater than that for ¹²³I.

We have no clear proof of any genetic effect from radiation in humans. Our estimates of genetic risk come from studies done years ago with rodents. Those studies showed that the risk from exposure of the mother was substantially less than that for the father. If there was an interval of at least eight weeks between exposure of the mother and conception, no adverse effects were seen in the offspring, even at rather high doses.

We further have no clear proof that there is any adverse effect from diagnostic medical exposure, including your scan. Neither can we state unequivocally that there is no risk. Some data even suggest that there can be beneficial effects from small doses of radiation.

Your body (including your milk) was radioactive immediately following your scan, but the activity cleared rapidly. Thus you were told to suspend breast-feeding for one week.

Conclusion: There is no convincing evidence of any risk from your getting pregnant at this time, and certainly none if you wait for a couple of months. There is further no convincing evidence of any risk to your small children.

S. Julian Gibbs, DDS, PhD