Martha Brower: Re your questions on Iodine-131: The use of radioactive iodine in breastfeeding mom is fraught with dangers. To begin with, iodine is sequestered in thyroid tissue at incredibly high levels. Most I-131 administered to the mom will go immediately to her thyroid, and the gamma irradiation will subsequently destroy the thyroid tissue... this is expected, its the reason we use it for this process. Very high doses are generally administered, and if you were to put a Gamma Counter to her thyroid, the dial will go crazy with activity. But the big problem with breastfeeding moms, is that Iodine(including I-131) is also trapped in milk AT HIGH LEVELS. Some reports show a milk:plasma ratio as high as 26, the highest I've ever seen for a drug. If the infant is breastfeeding during this exposure, the Iodine-131 would sequester in the infants thyroid, subsequently ablating it just like the mothers. So Iodine-131 is absolutely contraindicated in BFing moms. Iodine-131 has a radioactive half-life of 8.1 days. If you were to wait for 5 half-lives, 98% of the radiation would be gone. Some authors recommend that the mom wait for 50-52 days prior to reinitiating breastfeeding... I don't think thats too practical. Other authors strongly recommend that the mom quit breastfeeding altogether. If the mom were close to a university, she should take her milk, and have the I-131 determined by a gamma counter. Then the question is, what would be a safe background level. I don't know what that is, but probably 2-3 times the normal background would not prove detremental to the infant. As to your question, will I-131 use in a lactating mom increase her risk of breast cancer, I don't think anyone can answer that question. Just remember however, that the I-131 levels in the breast would be less than the thyroid, but probably quite significant. Hence, long-term gamma radiation exposure of the breast tissue would be likely, and hence could be the reason for the increase risk of breast cancer. I think extreme caution is required with I-131 ablation of the thyroid. *********************************** T.W. Hale, Ph.D. Associate Professor of Pediatrics Texas Tech University School of Medicine ***********************************