A patient who is pregnant should not be treated with RAI (7). RAI crosses freely into the placenta, and the fetal thyroid tissue is capable of accumulating iodine after the 12th week of gestation. Administration of RAI during this period may result in severe neonatal hypothyroidism. Fetal and neonatal thyroid irradiation may also increase the risk of developing thyroid cancer later. Retained activity in the maternal bladder may also increase the risk of malignancy by direct radiation exposure to the fetus.
Women who are actively lactating or nursing also should not receive RAI. Iodine is excreted in breast milk. NIS protein expression is also increased significantly in mammary tissue during lactation, thus increasing the radiation exposure to the breast. Nursing should not be resumed until the birth of the next child.
RAI has no role for the treatment of hyperthyroid conditions that are self-limited or when thyroid tissue is not hyperfunctioning. These conditions include silent, subacute, and postpartum thyroiditis in addition to factitious thyroid disease.
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