The thyroid gland needs iodine to produce thyroid hormones. The follicular cells of the thyroid have channels or pores to actively absorb iodine for making thyroid hormone. Radioactive iodine takes advantage of the fact that thyroid cells and thyroid cancer cells absorb iodine; therefore, it has been used to diagnose or treat various thyroid disorders.
Iodine is made into two radioactive isotopes, I-123 and I-131, that are commonly used in patients with thyroid disease. A radioactive isotope is a substance that gives off radiation. RAI is given by mouth, in pill or liquid form. RAI is then absorbed by the thyroid cells and can be used to diagnose or treat thyroid problems.
I-123 is most frequently used to take pictures of the thyroid gland. The dose of I-123 is small, and because of its short half-life (about 12 hours), there are fewer precautions that need to be taken. The disadvantage is that the small dose is not as sensitive at detecting thyroid tissue compared to the larger dose of I-131 that is used for treatment.
I-131 is commonly used to treat thyroid disorders, such as hyperthyroidism, through RAI ablation (medically destroying the thyroid gland without surgery). In addition, large doses of I-131 are used to destroy thyroid cancer cells, which are killed after absorbing the dose of radioactive iodine
No matter how expert the thyroid surgeon is, the majority of time there is a small amount of thyroid tissue that remains after thyroidectomy (removal of thyroid gland). For patients with a diagnosis of thyroid cancer, we use RAI to destroy remaining thyroid cells after surgery, a treatment called remnant ablation.
The goal of RAI treatment is to kill any remaining thyroid cells. This is important for several reasons:
The majority of patients will receive RAI remnant ablation; however, we strive to give the lowest dose possible and to identify patients who do not have evidence of remaining disease and thus would not benefit from receiving the RAI dose.
For patients who need RAI treatment, we only give additional doses of RAI after we determine that the thyroid cancer is growing (indicated by an increase in Tg levels) and if the remaining cancer cannot be surgically removed. There are a number of different radiologic studies and nuclear medicine studies that can be used to find remaining thyroid cancer. These techniques will be discussed with you and your child if and/or when necessary.
Prior to receiving RAI for treatment of thyroid cancer, patients must be placed on a low-iodine diet and their TSH level must be increased:
Note: All female patients of child-bearing age must have a pregnancy test before they are given a treatment dose of RAI. RAI, whether I-123 or I-131, should never be used in a patient who is pregnant or nursing.
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