Iodine-131 and Iodine-132
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.