Sodium Iodine I-131 is a radioactive material that is used to treat some diseases of the thyroid. It is one of several alternatives in the treatment of hyperthyroidism. It works by putting a high dose of radiation directly into the thyroid gland, without causing much radiation exposure to the rest of the body. The radiation destroys part of the overactive thyroid, which decreases the amount of hormone the thyroid makes.

I-131 is given in capsule form, and your body absorbs it from the GI tract. Once in the bloodstream, it is concentrated in the thyroid tissue, allowing the radiation to shrink the thyroid gland. As the thyroid shrinks, the hormone levels decrease, and your symptoms should improve. It can take 4-6 weeks to get the maximum effect, and may take as long as 3 months. Not all patients are good candidates for radioactive iodine treatment, and not all patients who get I-131 improve, although most do.

In order for the I-131 to have the maximum effect, the thyroid needs to be clear of outside iodine or thyroid medications. We may ask you to follow a low iodine diet for several days prior to treatment. If you are already on thyroid suppression such as PTU, this needs to be stopped for 2-3 weeks prior to treatment, and should not be restarted if necessary until at least 8 days after treatment. If you have been on thyroid replacement, your medicine may need to be stopped for 3-6 weeks before we can consider I-131 treatment. You must also wait 6 weeks for treatment if you have had an IVP or a CT scan where you received a contrast [dye] injection. This contrast contains iodine.

Although the amount of radioactivity is small, we do recommend the following precautions:

  1. Flush the toilet two or three times each time you urinate for 48 hours after getting the capsule, because your urine also contains a small amount of I-131. Drink lots of fluids, starting 4-6 hours after the treatment, to help flush excess I-131 out of your system.
  2. Do not share eating utensils, food, gum, tastes of food, licks of ice cream, etc for 1 or 2 weeks after the treatment, because your saliva will contain a small amount of radioactive iodine.
  3. After treatment, avoid close contact with pregnant women, infants, or toddlers less than 2 years old. This means arms-length distance for the first 2 weeks, on the lap only for the second 2 weeks, and prolonged contact with the neck only after 4 weeks.
  4. General family or work contacts pose no hazard, but intimate contact or contact with your body fluids [blood, urine, saliva] should be avoided for at least 1 or 2 weeks after the treatment. You are not a hazard to the general public in casual contact.
  5. You must not be pregnant at the time of I-131 treatment. If there is any risk that you might be pregnant, let your doctor know at the time of consultation. Your doctor may suggest you wait for 6 to 12 months before you try to become pregnant.
  6. If you are breast feeding, you must stop prior to I-131 treatment, as breast milk will transfer some of the radiation to the baby.


If ANY of the safety precautions will be difficult for you to follow, please inform your physician BEFORE you agree to the treatment.

The side effects from this treatment are relatively mild. Some patients experience a mild sore throat, difficulty swallowing, and occasionally some temporary swelling of the throat or salivary glands. You may take Tylenol for discomfort, but avoid aspirin, as this could interfere with the iodine uptake into the thyroid. You should take your other regular medicines as prescribed. A very uncommon side effect is thyroid storm, which happens if the body releases a large amount of thyroid hormone from the damaged gland. This is very rare, and you may be given a prescription for a beta-blocker, which reduces the side effects of thyroid storm to almost zero. The risk of developing a cancer from this radioactive treatment is very rare.

Although most patients respond to just one dose of I-131, some patients may require a second treatment at 3 months if their hormone levels have not gone down sufficiently. A few patients will still have symptoms after 2 treatments, and those patients may require surgery. It is also important to remember that 50-75% of patients may develop an underactive thyroid gland [hypothyroidism] after treatment. Sometimes this is temporary and many times this is permanent.