To:  My Colleagues
Re:  Iodine-131

There have been a number of comments regarding the use of Iodine-131 in a
breastfeeding mother that have been in error and I would like to correct
some misunderstandings re the pharmacology of this interesting ion.

Iodine-131 is specifically sequestered and concentrated in the thyroid.  The
thyroid gland has a membrane-specific pumping system that selectively pumps
iodine from the maternal plasma compartment into the patients thyroid...and
produces about at 40-50 fold concentration gradient in the  thyroid.  This
is why when you  administer I-131,  that about 45% of the dose 'will
concentrate in the thyroid', attaining high levels,  hence ablating or
destroying thyroid tissue.

Another major problem, is that about 40% of the dose will transfer into the
lactating breast alveoli(because it too has a pumping system),  so that the
breast tissue concentrates vast quantities as well.  Hence it has been
theorized that it might be dangerous for breastfeeding mother to use
radioiodine.

After the initial dose,  most of it will stay in the thyroid gland until it
is decayed away at the rate of one-half in 8 days (about 40 days for 97%
decay).  Some will distribute to the body, but only minimal amounts.

It is my contention,  that if a mom needs a thyroid ablation,  then she
should discontinue breastfeeding for long enough for her breasts to
discontinue making milk,  hence reducing the amount selectively transferred
into this tissue (thereby no "theoretical" risks to the breasts such as
breast cancer).  Cabergoline would probably be a suitable product to inhibit
lactation.

I know from personal experience,  that the thyroid emits huge amounts of
radiation for some time, which is the reason that the radiologist suggests
she not hold her child.  I once placed a gamma counter to the thyroid of
such a patient,  and it went off the meter (I regretted having done this as
the patient was quit concerned).  A thin lead blanket wrapped around the
thyroid would stop this.

So, things to remember are:  I-131 concentrates in thyroid tissue and the
lactating breast tissue.  It is slow to decay with an 8 day half-life.  If
the infant is breastfed,  it could produce severe thyroid problems,
including increasing the risk of thyroid carcinoma in later life.

Regards
Tom Hale, Ph.D.