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

***********************************