Radioiodine and pregnancy.

The fetal thyroid begins to accumulate radioiodine around the 12th week of pregnancy. Iodide easily crosses the placenta and fetal thyroid uptake can be effectively blocked by administration of radioiodine to the mother. Therapeutic administration of radioiodine to the mother will usually result in fetal hypothyroidism and may be associated with attention deficit disorders and impairment of figurative memory in the offspring. The hazard to a fetus from exposure to a family member, not the mother, who has been treated with radioiodine is very small and can be minimized by adherence to standard post treatment guidelines. Lactating mothers who have received standard diagnostic doses of 131I or who have been treated with 131I should not breast-feed their infant from that point forward. Breast-feeding in future pregnancies is not contraindicated. Pregnancy testing within 48 hours before 131I administration to potentially fertile women is a wise clinical practice. Demonstrated effects of radioiodine on spermatogenesis suggest that it is wise to recommend a 120-day waiting period between radioiodine and fertilization.