Radioactive Iodine (RAI) Imaging and Treatment
Why is radioactive iodine used to diagnose or treat thyroid disease?
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.
What is radioactive iodine?
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.
Radioactive iodine for thyroid imaging
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.
Radioactive iodine for treatment of thyroid disorders
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
Radioactive iodine for treatment of thyroid cancer
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:
- We assume any remaining cells are cancerous and by killing them we hope to reduce the chances that the cancer will grow back (decrease the chance of recurrence).
- Thyroid cells make a protein called thyroglobulin (Tg). If we can destroy all of the thyroid cancer cells, the Tg should be undetectable.
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:
- Low-iodine diet: Patients are placed on a low-iodine diet two weeks prior to receiving radioactive iodine. This makes them iodine-deficient and increases the chance that the thyroid cells will absorb the radioactive iodine dose.
- Increase the TSH level: A high thyroid-stimulating hormone (TSH) level increases the ability of thyroid cells to absorb iodine. TSH is secreted by an area of the brain called the pituitary gland, which tightly controls the amount of hormone produced by the thyroid gland.
Increasing the TSH level can be accomplished by stopping the thyroid hormone replacement for two to four weeks (thyroid hormone withdrawal; THw) or by giving a man-made form of TSH called recombinant human TSH (rhTSH) until the TSH reaches the desired level. Advantages and disadvantages of THw vs. rhTSH can be discussed with your provider.
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.
Ongoing care and research at CHOP
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