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Radioactive Iodine Uptake Test

Test Overview

A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The test can show how much tracer is absorbed by the thyroid gland and if it is evenly spread in the gland. This helps your doctor know if the thyroid gland is working properly. The radioactive tracer commonly used in this test is iodine.

A radioactive iodine uptake test is done to find problems with how the thyroid gland works, such as hyperthyroidism. An RAIU test may be done at the same time as a thyroid scan.

Why It Is Done

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

  • Find the cause of an overactive thyroid gland (hyperthyroidism).
  • Plan treatment for hyperthyroidism.

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 of the following medicines that can change the RAIU test results:
    • Thyroid hormones
    • Antithyroid medicines
    • Medicines that contain iodine, such as iodized salt, kelp, cough syrups, multivitamins, or the heart medicine amiodarone (such as Cordarone or Pacerone)
  • 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 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.
  • Have had any test using radioactive materials or iodine dye 4 weeks before the RAIU test. These other tests may change the results of the RAIU test.
  • Are or might be pregnant.
  • Are breast-feeding.

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

To prepare for an RAIU test:

  • Do not eat for 2 hours before the test.
  • Do not take any antithyroid medicine for 5 to 7 days 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.

Before a radioactive iodine uptake (RAIU) test, you need to sign a consent form that says you understand the risks of the test and agree to have it done. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is Done

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

For this test, you will lie on your back with your head tipped backward and your neck extended. It is 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 4 to 6 hours after you took the iodine. This is not an X-ray machine—it is a scanner that detects the radiation given off by the tracer. The test takes about 10 minutes. Another scan is done again in 24 hours.

After an RAIU test, you can do your regular activities. But you will be asked to take special precautions when you urinate. This is because your body gets rid of the radioactive tracer through your urine. This takes about 24 hours. It is important to flush the toilet and wash your hands thoroughly after each time you urinate.

How It Feels

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

Risks

There is always a slight chance of damage to cells or tissue from radiation, including the low levels of radiation used for this test. But the chance of damage from the radiation is usually very low compared with the benefits of the test.

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

Results

A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The radioactive tracer used in this test is iodine. An RAIU test is done to check for thyroid gland problems, such as hyperthyroidism.

Radioactive thyroid scan and radioactive iodine uptake test (RAIU)
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. The uptake may be even or uneven. If hyperthyroidism is present, abnormal test results may mean certain conditions are present.

  • A low uptake of tracer by the thyroid gland may mean that hyperthyroidism is caused by inflammation of the thyroid gland (thyroiditis), taking too much thyroid medicine, or another rare condition.
  • A high uptake of tracer spread evenly in the thyroid gland may mean that hyperthyroidism is caused by conditions such as Graves' disease.
  • An uneven spread of tracer in the thyroid gland (with either low or high areas of uptake) may mean that hyperthyroidism is caused by a multinodular goiter or a noncancerous (benign) tumor called a toxic adenoma.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking thyroid medicine.
  • Eating foods with iodine, such as shellfish, iodized salt, or kelp.
  • Having other tests using contrast materials in the past 4 weeks.

What To Think About

  • Blood tests may be done before a radioactive iodine uptake (RAIU) to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood. For more information, see the topics Thyroid-Stimulating Hormone (TSH) and Thyroid Hormone Tests.
  • A thyroid scan may be done at the same time as an RAIU test. For more information, see the topic Thyroid Scan.

References

Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Matthew I. Kim, MD - Endocrinology
Last Revised May 31, 2011

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