The thyroid is one of the few organs in the body that has the ability to absorb iodine. The absorbed iodine is used by the thyroid to make thyroid hormone. Understanding this has allowed physicians to use radioiodine safely for nearly 100 years. Radioiodine can be used in small tracer amounts to create an image of the thyroid or thyroid remnant after surgery (radioactive iodine scan) or it can be used in higher doses to ablate (destroy) thyroid tissue of thyroid cancer. Salivary glands also absorb small doses of iodine, which is why some patients treated with high doses of radioiodine can experience a dry mouth. The radioiodine that is not taken up by thyroid cells is eliminated from the body, mostly in the urine. Using radioiodine is a safe and effective way to test and treat thyroid conditions and has not been associated with an increased risk for thyroid cancer or any other type of cancer.
Radioactive Iodine As a Diagnostic Tool
Radioactive Iodine Scanning is a diagnostic test in which a small tracer dose of radioactive iodine (I-131 or I-123) is given by mouth. The drug is taken up by the thyroid which then emits a small amount of radioactivity that can be detected by a geiger counter to create an image. This test identifies the size and location of the thyroid gland. It can also be used to identify sites of thyroid cancer that have spread beyond the thyroid itself. The test is commonly used to evaluate the function of the thyroid and can help in the evaluation and diagnosis of patients with subacute thyroiditis, Graves’ disease and toxic nodular goiters. In the past, radioactive iodine scanning was used to evaluate thyroid nodules. “Cold” nodules were associated with tumors, while “hot” nodules indicated a focus of thyroid tissue that was overproducing thyroid hormone. Radioactive iodine scanning is still helpful in the workup of patients with hyperthyroidism, but the evaluation of most thyroid nodules today is better done with fine needle aspiration.
Radioactive Iodine As a Treatment Tool
Radioactive iodine can also be used to treat the hyperthyroidism associated with Graves’ disease and toxic nodular goiters and to ablate any remaining thyroid tissue or thyroid tumor cells in patients who have undergone surgery for thyroid cancer.
Although some patients with Graves’ disease experience a spontaneous remission of their symptoms, most patients will require definitive therapy. This can often be achieved by treating the patient with radioactive iodine, which destroys the thyroid cells that are overproducing thyroid hormone. Most patients will need just a single treatment, while about 10 percent of patients will require a second dose. The treatment takes several months to work, and patients are typically rendered hypothyroid and will need to take thyroid hormone replacement therapy. Radioactive iodine can also be used to treat patients with nodular goiters that overproduce thyroid hormone in a similar fashion.
Patients who undergo a total thyroidectomy for thyroid cancer may benefit from a dose of radioactive iodine following the surgery. This is typically done 6 to 8 weeks following surgery. The treatment will destroy any remaining normal thyroid cells and will reduce the chances of tumor recurrence in certain patient populations. Destroying any remaining thyroid tissue also aids in the routine follow up of patients with thyroid cancer. A protein called thyroglobulin is made by both normal thyroid cells and by thyroid tumor cells. After a total thyroidectomy and radioactive iodine ablation, the blood thyroglobulin level should be undetectable. A rising thyroglobulin level is concerning for tumor recurrence and could be treated with additional doses of radioactive iodine or further surgery depending on the site of disease.
For radioactive iodine to be effective, the thyroid stimulating hormone (TSH) level must be elevated. This can be achieved in two ways:
Thyroid Hormone Withdrawal
TSH is made by the pituitary gland in the brain in response to low levels of circulating thyroid hormone. In patients who have had their thyroid surgically removed, an increase in the TSH (a level of at least 30 is needed for effective treatment) can be achieved by withholding thyroid hormone replacement therapy. This will make the patient hypothyroid, and will stimulate the pituitary to release high levels of TSH. It typically takes about 6 weeks for the TSH to rise to the necessary level. As the thyroid hormone levels fall, patients will feel fatigued. The hypothyroid symptoms can be ameliorated by treating the patient with a short acting form of thyroid hormone called Cytomel. But even this must be discontinues two weeks before the treatment dose of radioactive iodine is administered.
Thyrogen is a synthetic form of TSH that can be injected to artificially increase the TSH levels. Injections are given on the two days prior to administration of the dose of radioactive iodine. The advantage of this approach is that the patient does not become hypothyroid will all of the attendant symptoms. The drawback is the cost, which may or may not be covered by insurance companies.
Both techniques appear to be equally effective, but endocrinologists may recommend one approach over another depending on the particular circumstances of an individual patient. It is important to discuss with a thyroid specialist which form of preparation is best for you. In addition, patients should be on a low iodine diet for 2 weeks before the RAI therapy.
Patients usually do not need to be hospitalized after receiving a dose of radioactive iodine which exits the body primarily in the urine, but also in the sweat and saliva. Patients who receive radioactive iodine as an outpatient need to follow several precautions for 5 days following administration of the drug:
- Patients receiving radioactive iodine should avoid
- Close contact (< 3 feet) with young children, elderly, and pregnant women
- Sharing a toilet or bathroom. Flush with the lid down.
- Sharing food.
- Sharing a bed.
- Kissing and sexual activity.
- All clothes, towels, and linens should be washed separately.
- Wash hands and the rest of the body often.
Radioactive iodine is absorbed by thyroid tissue and by the salivary glands. While the treatment is generally well tolerated, some patient will experience nausea and vomiting which is self-limited. Because radioactive iodine is also absorbed by the salivary glands, some patients will develop swelling under the tongue or within the neck. Treatment can also lead to a dry mouth which general improves with time. Patients report symptomatic relief by sucking on hard candy and drinking plenty of fluids. Pregnant and breast feeding women should never receive radioactive iodine which can destroy the thyroid of a fetus or in the infants of breast feeding women.