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Outcome of Pregnancy After Exposure to Radioiodine In Utero.

OBJECTIVE
Radioiodine (131I) therapy is absolutely contraindicated in pregnancy yet reports of inadvertent exposure continue to appear in the literature. In this review, we discuss the risks of fetal exposure and prevention strategies in the light of current guidelines.


METHODS
We performed a literature search on MEDLINE using the terms radioiodine, I-131, toxicity, complications and pregnancy and chose the most relevant studies for this review.


RESULTS
Before implantation, the major concern is miscarriage and death of the embryo above a radiation threshold of 100mGy (10 rads). Exposure to 131I at this very early stage of pregnancy is unlikely to result in major malformations or thyroid dysfunction in surviving embryos. Exposure later in pregnancy i.e. during thyroidogenesis (from 10 weeks gestation) and organogenesis (from 2 weeks gestation) at similar radiation thresholds may result in fetal thyroid ablation, birth defects and in later life, growth retardation and reduction in IQ. In addition to these deterministic effects, radiation at any dose may increase the risk of cancer (stochastic effect) and recent evidence indicates an increased risk of thyroid cancer many years after in utero exposure.


CONCLUSIONS
Clinicians treating women of child-bearing age with radioiodine need to be aware of the risks of fetal exposure to radioiodine and take all measures to avoid inadvertent exposure during pregnancy.