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I-131 for Thyroid Cancer

Diagnosis of Thyroid Cancer and Treatment with I-131

What is radioiodine I-131 treatment?

Radioiodine I-131 treatment is a time-tested and effective treatment for the two most common types of thyroid cancer – papillary and follicular. Most people have radioiodine I-131 for thyroid cancer after having surgical removal of all or part of the thyroid or for recurring thyroid cancer. Treatment with radioiodine I-131 can find remnant and metastasized cancer and destroy it by targeting the cancer cells and eliminating them wherever they may be.

What types of thyroid cancer are treated with radioiodine I-131?

Thyroid cancer is a rare type of cancer that starts in the cells of your thyroid gland. There are different types of thyroid cancer cells that are identified by a laboratory examination of biopsied tissue under a microscope. Two of these types of cancer, papillary and follicular thyroid cancer, take up iodine and can be treated with radioiodine I-131.

  • Papillary thyroid cancer
    This is the most common type of thyroid cancer, accounting for about 80% of cases. It usually grows slowly and is more common in women and younger people.
  • Follicular thyroid cancer
    This type is less common than the papillary variety, accounting for about 15% of thyroid cancers. It tends to grow faster and is more likely to spread to other parts of the body. It is more common in older people and in regions of the world where iodine intake is low.

Medullary and anaplastic thyroid cancers are uncommon varieties of thyroid cancer that do not take up iodine and require other forms of post-surgical treatment, such as chemotherapy, targeted therapy, or immunotherapy.

Diagnosis of thyroid cancer

Thyroid cancer can be discovered in several ways:

  • A health care provider may feel a nodule or swelling during a physical examination of the neck and thyroid gland.
  • A blood test can measure the levels of thyroid hormones and other factors.
  • While most thyroid nodules do not cause symptoms, you may experience:
    • A lump or swelling in the neck.
    • Changes in voice, such as hoarseness or difficulty speaking.
    • Trouble swallowing or breathing.
    • Pain in the neck or throat.
    • Swollen lymph nodes in the neck.
  • Imaging tests, such as ultrasound, CT scan or MRI, are often performed to check the size and shape of the thyroid gland and look for any nodules or abnormal growths. Some thyroid cancers are found when a patient has an imaging exam for another reason and the radiologist notices a mass or nodule on the thyroid. This is called an incidental finding.

A fine-needle aspiration biopsy will confirm if and what type of thyroid cancer you have. This involves taking a small sample of tissue from the thyroid nodule or mass and examining it under a microscope. While thyroid nodules are common, they are usually not cancerous.

Treatment for thyroid cancer

After your medical team determines the type and stage of thyroid cancer you have, there are several options for treatment, including surgery to remove all or part of the thyroid, radiation, chemotherapy, hormone therapy, and targeted therapy. Your cancer care team will guide you on the best treatment for your cancer. For a detailed explanation of types of thyroid cancer and their treatment, please visit:

After you have completed your initial treatment for papillary or follicular thyroid cancer, your cancer care provider may recommend treatment with radioiodine I-131, a well-established and widely used treatment for hyperthyroidism and thyroid cancer since its introduction in the 1940s. I-131 can find and eliminate thyroid cancer cells throughout the body that might have been missed by other treatments.

Treatment for thyroid cancer with radioiodine I-131

Radioiodine (radioactive iodine) therapy is a molecular radiology treatment that uses a radioactive form of iodine called iodine-131 (I-131) to destroy remaining thyroid cancer cells after surgery. The thyroid gland and thyroid cancer cells absorb nearly all the iodine in your body, so radioiodine I-131 can target them specifically without affecting other organs.

Radioiodine I-131 treatment can extend lives and improve quality of life for people with papillary or follicular thyroid cancer that has spread to the neck or other parts of the body (metastasized).

How does radioactive iodine therapy work?

Radioiodine therapy works by taking advantage of the fact that thyroid cells are very efficient at absorbing and storing iodine from your diet. When you swallow a capsule or liquid containing I-131, it travels into your bloodstream and reaches your thyroid gland. There, it is taken up by the thyroid cancer cells and emits radiation that damages or kills them. I-131 can also find metastasized thyroid cancer cells throughout the body and eliminate them.

The amount of radiation delivered by I-131 depends on the dose you receive and how much iodine your thyroid cells take up. The dose is calculated based the type and extent of your thyroid cancer, as well as other factors. Your healthcare provider will monitor your response to the treatment using blood tests and scans.

The radiation from I-131 is very targeted, so it mainly affects your thyroid gland, nearby tissues, and other thyroid cancer cells in your body. Most of the excess I-131 that is not absorbed by your thyroid cells is eliminated from your body through urine within a few days.


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Treatment for thyroid cancer is given on an outpatient basis at one of ARA’s imaging centers that performs nuclear medicine procedures.

You will go to the imaging center on the day of your treatment to receive a single pill of radioiodine I-131 which you take by mouth. You will then be able to go home.

As the radioiodine moves into your bloodstream and through your thyroid gland, it concentrates in thyroid gland cells throughout the body and begins destroying thyroid cancer cells.

Most people who receive radioiodine I-131 need only one dose.

You may have a few short-term side effects, including:

  • Tenderness and swelling around the thyroid
  • Swelling of the salivary glands
  • Dry mouth
  • Changes in taste
  • Dry eyes

Women should avoid becoming pregnant for 6 to 12 months after the treatment.

You will need to have follow-up visits with your healthcare provider to check your thyroid function and overall health. You may need to have yearly whole-body iodine scans and blood tests to check for any recurrence of cancer.

  • Your referring health care provider may advise you to follow a low-iodine diet for a week prior to the procedure to maximize the absorption of I-131 when you ingest it.
  • Your healthcare provider will instruct you to stop taking anti-thyroid drugs at least three days before the procedure and stop taking thyroid hormone replacement medication five to seven days before the procedure. The purpose of this is to raise your thyroid stimulating hormone (TSH) levels so that your thyroid tissue and cancer cells will take up more of the I-131. Alternatively, you may need injections of synthetic TSH (Thyrogen). If you receive an injection, you will not have to stop taking your medication.
  • You should not eat or drink 2 to 4 hours before and 1 hour after taking I-131.
  • If you are a biological female of childbearing age, a serum pregnancy test MUST be done within 7 days before administration of the I-131 dose.
  • If you are breastfeeding a child, you will need to stop immediately after taking I-131 and you will not be able to resume breastfeeding this child. You will be able to breastfeed any children you may have in the future.


  • It is a simple and effective treatment that can be done on an outpatient basis.
  • It has few side effects and does not affect other organs or tissues.
  • It can reduce the chance of metastasis by eliminating cancer cells that remain after surgery.


  • It may affect the salivary glands and cause dry mouth or taste changes.
  • It requires precautions to avoid exposing others to radiation for a few days after treatment, such as avoiding close contact with others.

Having radioiodine I-131 therapy is a decision that should be made in consultation with your healthcare provider and your radiologist, who will advise you about your individual situation and the best course of action.

Follow directions on your patient discharge instructions to safely manage the radiation in the radiopharmaceutical as it passes out of your body in your urine. This will protect you, as well as family members and caregivers around you. If you need medical care within the first few days after your radiopharmaceutical infusion, be sure and tell your providers that you have been treated with radioiodine I-131. There may still be some radiation in your body, and your healthcare providers should take universal precautions. Most of the radiation will leave your body within 3 days.

Basic instructions for protecting others around you after I-131 treatment

  • Stay at home, isolated from the general public, for 3 days.
  • Maintain a distance of 6 feet from others for 3 days.
  • Sleep alone for 5 days.
  • Wait 3 days before returning to work, then you may return if you feel well enough.
  • Have sole use of a bathroom, if possible, for 4 days. Wash hands thoroughly.
  • Men should sit when urinating.
  • Flush 2 to 3 times after each use.
  • Drink plenty of fluids for the first few days and urinate frequently to keep the bladder empty.

Interacting with pets

  • Pets can be affected, too. You may want to take the same precautions with them.

Driving home from the theranostics center

  • The closest you should be to someone is 3 feet. So, if your spouse or caregiver is driving, consider sitting in the back seat.

After your procedure, you will receive ARA’s Instructions for Outpatient Treatment with Iodine-131. Your medical team will review this with you and have you sign a Patient Agreement before you leave the imaging center.

Your healthcare provider will refer you to ARA for scans and treatment, either by giving you a paper referral or through an electronic medical system. You should receive an automated call from ARA if your referral is sent electronically. Please call (512) 453-6100 to schedule your appointment.