To      : Elisa Hirsch
Re      : Thyroid Scan

        A thyroid scan can actually be one of two different tests.  In one
procedure,  the radiologist uses radioactive Technitium-99m pertechnate
which has a short half-life of only 6.7 hours.  At least 97.5% of the
radioactivity would be gone in 5 half-lives(33.5 hours).  It would probably
be quite safe to breastfeed after this interval.

        In the second procedure(an uptake scan)  radioactive Iodine-131 is
used. The radioactive half-life of I-131 is 8.1 days.  Five half-lives in
this situation is 40.5 days.  A great deal of controversy exists over how
long a mother should discontinue breastfeeding after taking I-131.  I
personally believe it will be many weeks before the breast milk
radioactivity will be down to background levels.  The best answer is to
remove milk samples after several weeks of dumping,  and simply have them
counted in a geiger counter in a clinical laboratory.

        Excessive exposure of the pediatric thyroid to I-131 could easily
destroy the gland completely. So extreme caution is urged in this
situation.


Tom Hale