Why It Is Done

A radioactive iodine uptake (RAIU) test is most often done to:

• Find the cause of an overactive thyroid gland (hyperthyroidism).
• Plan treatment for hyperthyroidism.
• Plan treatment for patients who have had thyroid cancer surgery.

How To Prepare

Tell your doctor if you:

• Take any medicines regularly. Be sure your doctor knows the names and doses of all your medicines. Your doctor will instruct you if and when you need to stop taking any medicines that can change the RAIU test results, including:
  • Thyroid hormones.
  • Antithyroid medicines.
  • Medicines or supplements that contain iodine, such as iodized salt, kelp, cough syrups, multivitamins, and the heart medicine amiodarone.
• Are allergic to any medicines, such as iodine. But even if you are allergic to iodine, you will likely be able to have this test. That's because the amount used in the tracer is so small that your chance of an allergic reaction is very low.
• Have ever had a serious allergic reaction (anaphylaxis) from any substance, such as the venom from a bee sting or from eating shellfish.

Tell your doctor if you are or may be pregnant.

If you are breastfeeding, ask your doctor or radiologist if you will need to pause breastfeeding after the test. You may want to pump enough breast milk before the test to get through 1 to 2 days of feeding. Certain radioactive tracers used in this test can get into your breast milk and are not good for the baby.

Before an RAIU test, blood tests may be done to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood.

Follow your doctor's instructions about not eating before the test. Your doctor may ask you to eat a low-iodine diet.

For an RAIU, you will swallow a dose of radioactive iodine. Iodine can be taken as a capsule or a fluid 4 to 24 hours before the test. Iodine has little or no taste.

Just before the test, you will remove your dentures (if you wear them) and all jewelry or metal objects from around your neck and upper body.

How It Is Done

A radioactive iodine uptake (RAIU) test is done in the nuclear medicine section of a hospital's radiology department.

For this test, you will lie on your back with your head tipped backward and your neck extended. It's important to lie still during this test. A special machine is placed over your thyroid gland to measure the amount of tracer absorbed by the thyroid gland. This isn't an X-ray machine—it's a scanner that detects the radiation given off by the tracer. This test takes less than 30 minutes. It's done 4 to 6 hours after you are given the tracer. Another scan may be done in 24 hours.

After an RAIU test, you can do your regular activities.

How It Feels

You may find it uncomfortable to lie still with your head tipped backward.

Risks

Allergic reactions to the radioactive tracer are very rare.

Anytime you're exposed to radiation, there's a small chance of damage to cells or tissue. That's the case even with the low-level radioactive tracer used for this test. But the chance of damage is very low compared with the benefits of the test.

Most of the tracer will leave your body through your urine or stool within a day. So be sure to flush the toilet right after you use it, and wash your hands well with soap and water. The amount of radiation in the tracer is very small. This means it isn't a risk for people to be around you after the test.

This test is not usually done for pregnant women because of the chance of exposing the baby (fetus) to radiation. This test is also not recommended for breastfeeding women or young children.

Results

Normal:

• The amount of radioactive tracer in the thyroid gland is normal. An RAIU test measures the amount of tracer taken up by the thyroid gland at certain times after the tracer is given. The measured amount of radioactive tracer in the thyroid gland at each one of these times is at normal levels.

Abnormal:

• The test shows either more or less uptake of tracer than normal in the thyroid gland. If hyperthyroidism is present, abnormal test results may mean that certain conditions are present.