iodine uptake in thyroid -Reply

[Charles Willis <CAW@nrc.gov>]

Drew,

You are into a tricky area.  The numbers you see published are for high specific
activity iodine (e.g. I-131) and are based on an assumption of "normal" intake of
stable iodine.  "Normal" intake of stable iodine has changed over the years.  The
thyroid does adjust uptake as a function of intake, but the relationship is not
simple.  If  there is an under-supply of iodine, the thyroid responds by developing
colloid goiter.  Conversely, if there is an over-supply of stable iodine, the thyroid
develops nodules (nodular goiter).  The thyroid seems to deal with iodine intakes
in the 40 to 80 micrograms per day range.  This might be considered the normal
range but the normal range often is taken as what the public is doing at the time
and today, the average intake may be higher.  

For low specific activity I-129, the problem is different from that with high specific
iodine because the I-129 may constitute a significant fraction of the mass.   The
problem might be approached  by determining the specific activity of the I-129 in
the total iodine consumed.  In time (perhaps assume the ICRP-30 biological half
life of 120 days) the specific activity of the iodine in the thyroid will be the same as
that in the intake.  The thyroid might contain some 2 to 3 milligrams of iodine.  

Of course, this is simple modeling.  If you want an accurate answer you must make
measurements, in part because human variability is great.

Have fun.

Charlie Willis
caw@nrc.gov