Re: iodine uptake in thyroid

[Jocelyn Towson <jtowson@nucmed.rpa.cs.nsw.gov.au>]

>I'm looking for some information on the uptake of iodine in the thyroid
>for adults (chronic exposures), more specifically, the relative or
>fractional uptake of iodine when compared to the normal amount of iodine
>ingested.  ICRP 56 uses the value of 0.3.  Dunning and Schwarz (Health
>Phys 40 (5) 1981) provide estimates of the distribution of iodine uptake
>which shows that the mean is 0.19, median 0.17, and the mode 0.15.  The
>value of 0.3 probably represents about the 95% upper bound.  I'm
>uncertain as to whether this value is appropriate for chronic exposures
>to radioactive iodine (I-129) that represent anywhere from 50% to 200% of
>the normal, daily requirements of iodine (roughly 150 Ęg/d ingested -
>30 Ęg used (Guyton and Hall - Textbook of Medical Physiology)).  Could
>the fractional uptake simply be estimated as the amount needed over the
>amount ingested, i.e., does the body maintain the iodine concentration in
>the body under homeostatic control?
>
>Any insight, references, etc., would be appreciated.

There is a nice graph of fractional iodine uptake as a function of dietary
iodine intake which is reproduced in an article in Health Physics 46:
1265-1279, 1984.  The article is by D Crocker entitled Nuclear reactor
accidents - the use of KI as a blocking agent against radioiodine uptake in
the thyroid - a review.  The source of the graph is acknowledged as a paper
by Kaul et al in Radiat. Environ. Biophys. 18: 185-195, 1980.
The fractional uptake is shown as a maximum of around 0.8 for low iodine
intakes of about 20 micrograms/day, falling to 0.3 at about 140 and 0.15
around 300 micrograms/day.  That's the reason some centres recommend their
thyroid cancer patients go on a low iodine diet before radioiodine therapy.
Regards,
Jocelyn Towson

Jocelyn Towson, RSO
Dept of PET & Nuclear Medicine
Royal Prince Alfred Hospital
Camperdown, NSW 2050
Australia

tel [national]  02 9515 8011    [international] 61 2 9515 8011
fax [national]  02 9515 6381    [international] 61 2 9515 6381
email   jtowson@nucmed.rpa.cs.nsw.gov.au