Side Effects of Radioactive Iodine | UCLA Endocrine Center

Radioactive Iodine (RAI) in the treatment of hyperthyroidism and thyroid cancer

Radioactive iodine (RAI) ablation therapy, also known as radioactive iodine treatment or iodine-131 therapy, is a medical procedure used primarily in the treatment of certain thyroid conditions, particularly thyroid cancer and some cases of hyperthyroidism. Here is an overview of the key aspects of RAI ablation therapy:

What is radioactive iodine (I-131)?

Iodine is an essential component for the production of thyroid hormones. I-131 is a radioactive isotope of iodine that has a half-life of 8 days and emits radiation in the form of gamma rays and beta particles.  When taken orally, it is selectively taken up by thyroid cells (including most types of thyroid cancer cells), where the emitted radiation then destroys the cells.

Who should receive RAI?

Patients with Graves Disease

Graves' disease is an autoimmune disorder that leads to overactivity of the thyroid gland.  It is the most common cause of hyperthyroidism. Radioactive iodine (RAI) therapy is a common treatment option for Graves' disease.

  • RAI is a targeted treatment that takes advantage of the thyroid gland's ability to concentrate iodine. The radioactive iodine is absorbed by the overactive thyroid cells, leading to their destruction.
  • The primary goal of RAI in Graves' disease is to reduce the excess production of thyroid hormones (T3 and T4) and bring the thyroid function back to normal levels. This helps alleviate symptoms of hyperthyroidism. After RAI treatment for Gravesdisease, most patients eventually develop hypothyroidism, which is then treated with levothyroxine (thyroid hormone in the form of a pill taken by mouth daily).  In >90% of cases, treating hypothyroidism with levothyroxine results in consistent levels of thyroid hormone in the bloodstream and healthy metabolism.  Treating hypothyroidism is generally easier than treating hyperthyroidism because disease thyroid tissue in hyperthyroid patients often secretes unpredictable amounts of thyroid hormone that fluctuate from day to day.

Patients with Thyroid Cancer

RAI ablation is commonly used after surgery for differentiated thyroid cancer.  About 95% of thyroid cancers are differentiated thyroid cancers (papillary thyroid cancer and follicular thyroid cancer), meaning that the cancer cells bear a strong resemblance to normal thyroid cells.  Differentiate thyroid cancers retain the ability to take up radioactive iodine and secrete thyroglobulin, a protein precursor to thyroid hormone that is present in the bloodstream and can be used as a tumor marker.  Currently, about 25% of patients who undergo surgery for differentiated thyroid cancer at our center receive RAI after surgery.  These are patients who are at intermediate to high risk of recurrence, meaning because their cancers showed the tendency to invade adjacent organs or metastasize outside the thyroid, they are more likely to persist in the body or return in the future.

Radioactive iodine was the original magic bullet” for cancer treatment, as it is taken up only by thyroid cells, thereby minimizing off-target effects to other normal tissues.  Taken after total thyroidectomy, the goal of RAI treatment is to (a) destroy/ablate any remaining normal thyroid tissue (even after very thorough surgery, tiny amounts of normal thyroid tissue may remain), and (b) kill thyroid cancer cells, with the caveat that RAI is only effective in killing microscopic areas of cancer cells (gross or visible disease must still be treated with surgery in most cases).

Preparing for radioactive iodine treatment

Patients may be asked to follow a low-iodine diet for several weeks before undergoing radioactive iodine treatment.  Additionally, levels of TSH (thyroid-stimulating hormone) in the blood must be raised to prepare for RAI treatment.  This can be done in two ways:

Thyroid hormone withdrawal.  In this method, the patient is instructed to stop taking levothyroxine for about one month.  The half-life of thyroid hormone is about 7 days. 

Recombinant human TSH (rhTSH) injection.


In California, RAI is typically administered in a capsule or liquid form from the hospital. The patient ingests the radioactive iodine at the hospital, and the substance is absorbed by the thyroid cells. After receiving RAI, patients may be required to isolate themselves for a period to limit radiation exposure to others. This is particularly important in cases of thyroid cancer where higher doses may be used.

Targeted Destruction

The radioactive iodine emits radiation that selectively targets and destroys thyroid tissue, including cancerous cells. Healthy thyroid tissue and other tissues in the body are less affected.

Monitoring and Follow-Up

Patients are monitored after RAI to assess their response to treatment. Follow-up may include thyroid function tests, imaging studies, and surveillance for recurrence or metastasis in the case of thyroid cancer.

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