Therefore, the thyroid gland, which takes up iodine to make thyroid hormone, cannot distinguish between the two. However, the nucleus of a radioactive iodine molecule has excess energy and gives off radiation that can have effects on the cells in which it is concentrated.
Radioactive iodine is given either in a capsule or in a tasteless solution in water.
Radioactive iodine treatment of hyperthyroidism makes use of the thyroid gland's natural need for iodine to make thyroid hormone. The thyroid is the only part of the body that collects and retains iodine. In hyperthyroidism, the thyroid cells are over-stimulated and make larger amounts of thyroid hormone. The excess amount of hormones are secreted into the blood, and produce the symptoms of hyperthyroidism. When radioiodine is given, the thyroid gland cannot tell if the iodine is radioactive or not, and collects it in the normal way in proportion to the activity of the thyroid.