By Pablo Barillas
May 3rd 2011
Dr. Mason
Both I-131 and I-125 are
radioactive isotopes of
naturally occurring I-127.
I-131 is highly radioactive
and gives of energy in both
beta and gamma radiation.
I-131 has a half-life of 8 days
and decays into stable Xenon
Created by nuclear fission of
either uranium or plutonium.
I-125 has electron capture
radiation decay where a
proton will capture an
electron and give off a
I-125 has a half-life of 59
days will decay into
Tellurium 125.
I-125 is created by the
electron capture decay of
Xenon 125.
Iodine as a molecule is
nonpolar and is highly soluble
in non polar organic solvents.
Due to its nonpolar nature, it
does not dissolve well in water.
Solubility is aided by iodide
ions already in the water, such
as from HI, KI etc and will
form the triiodide ion, I3−
Iodine is easily oxidized and
easily reduced as well.
I-131 was discovered by
Glenn T. Seaborg and John
Livingood at the University
of California Berkeley in the
late 1930's.
I-131 was a huge issue postWWII both in Japan and
nuclear test areas in the USA.
Currently I-131 is a source of
major concern at the
Fukishima nuclear plant.
I-131, even though harmful
in large exposures, has some
medicinal uses.
I-131 is currently used as
treatment for hyperthyroidism by destroying
tissue with radiation.
Sometimes also used as a
radioactive label, but not
often since it will destroy
tissue in the process.
I-125 which is less harmful
has more medicinal uses than
I-125 is used in
brachytherapy which aids in
the destruction of cancerous
I-125 is also used in radiology
by tagging antibodies in a
I-131 is a nuclear fission product so
anywhere fission takes place, I-131
can be found.
Fuel rods in nuclear reactors can crack
and release I-131 into the surrounding
cooling water, which can be released
into the environment.
Another major source of I-131 is
nuclear waste, if not correctly sealed
will leak into the environment.
A stated before, atomic bomb testing
released a lot of I-131 into the
Industrial, medical and university
waste can also be sources of I-131.
I-131 is easily assimilated into
the food web by uptake in
primary producers such as
Shellfish can then
bioaccumilate I-131 by filter
Vertebrates can absorb I-131
readily by the thyroid gland.
Due to radiation, I-131 will
lead to tissue destruction and
As stated before, ingestion is
the main mode of entry for
most organisms.
If there is a recent explosition
or emission of radioactive
products, I-131 maybe
inhaled as well.
Once in the body, the
sodium-iodide symporter,
located in the outer cells of
the thyroid will actively
transport iodide ions inside.
Stimulated by TSH, the
thyroid now begins
production of
Triiodothyronine (T3) and
Thyroxine (T4).
To do this, the enzyme
thyroperoxidase (TPO)
oxidizes the iodide anion and
incorporates it into
thyroglobin which will later
become T3 or T4.
Most of I-131 can be
eliminated in the urine.
If the organism stops the
intake of radioactive iodine, it
can let the short half-life of
the isotope decay it away.
To prevent I-131 uptake
entirely, it is possible to
saturate the thyroid with
stable I-127.
This can be done with KI
“Radioactive I-131 from Fallout”
“EPA: Radionucleotides, Iodine”
“Radiotoxicity of Iodine-123, Iodine-125, and Iodine- 131”
“Thyroid Disorders”
“Seaweed as a model for iodide uptake and retention in the thyroid”

iodine 131/125 - California State University, Long Beach