Iodine, as was mentioned earlier, concentrates in the thyroid gland, and
is converted there to protein-bound iodine that is slowly released to
the blood stream. Iodine-131, in concentrations much higher than those
used in diagnostic tests, will irradiate thyroid cells, thereby damage
them, and reduce the activity of an overactive thyroid
(hyperthyroidism). The energy is released within the affected gland, and
much of it is absorbed there. Iodine-131 has a half-life of 8.1 days. In
contrast, ¹³²I has a half-life of only 2.33 hours. What this means is
that the same weight of radioactive ¹³²I will give a greater radiation
dose than ¹³¹I would, and lose its activity rapidly enough to present
much less hazard by the time the iodine is released to the blood stream.
Iodine-132 is therefore often preferred for treatment of this sort.