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Radioactive iodine uptake

Iodine uptake test; RAIU

 

Radioactive iodine uptake (RAIU) tests thyroid function. It measures how much radioactive iodine is taken up by your thyroid gland in a certain time period.

A similar test is the thyroid scan. The 2 tests are commonly performed together, but they can be done separately.

How the Test is Performed

 

The test is done in this way:

  • You are given a pill that contains a tiny amount of radioactive iodine. After swallowing it, you wait as the iodine collects in the thyroid.
  • The first scan is usually done 4 to 6 hours after you take the iodine pill. Another scan is usually done 24 hours later. During the scan, you lie on your back on a table. A device called a gamma probe is moved back and forth over the area of your neck where the thyroid gland is located.
  • The probe detects the location and intensity of the rays given off by the radioactive material. A computer displays images of the thyroid gland.

The test takes about 30 minutes.

 

How to Prepare for the Test

 

Follow instructions about not eating before the test. You may be told not to eat after midnight the night before your test.

Your health care provider will tell you if you need to stop taking medicines before the test that may affect your test results. DO NOT stop taking any medicine without first talking to your provider.

Tell your provider if you have:

  • Diarrhea (may decrease absorption of the radioactive iodine)
  • Had recent CT scans using intravenous or oral iodine-based contrast (within the past 2 weeks)
  • Too little or too much iodine in your diet

 

How the Test will Feel

 

There is no discomfort. You can eat beginning about 1 to 2 hours after swallowing the radioactive iodine. You can go back to a normal diet after the test.

 

Why the Test is Performed

 

This test is done to check thyroid function. It is often done when blood tests of thyroid function show that you may have an overactive thyroid gland.

 

Normal Results

 

These are normal results at 6 and 24 hours after swallowing the radioactive iodine:

  • At 6 hours: 3% to 16%
  • At 24 hours: 8% to 25%

Some testing centers only measure only 24 hours. Values may vary depending on the amount of iodine in your diet. Normal value ranges may vary slightly among different labs. Talk to your provider about the meaning of your specific test results.

 

What Abnormal Results Mean

 

Higher-than-normal uptake may be due to an overactive thyroid gland. The most common cause is Graves disease.

Other conditions can cause some areas of higher-than-normal uptake in the thyroid gland. These include:

  • An enlarged thyroid gland that contains nodules producing too much thyroid hormone (toxic nodular goiter)
  • An single thyroid nodule that is producing too much thyroid hormone (toxic adenoma)

These conditions often result in normal uptake, but the uptake is concentrated into a few (hot) areas while the rest of the thyroid gland does not take up any iodine (cold areas). This can only be determined if the scan is done along with the uptake test.

Lower-than-normal uptake may be due to:

  • Factitious hyperthyroidism (taking too much thyroid hormone medicine or supplements)
  • Iodine overload
  • Subacute thyroiditis (swelling or inflammation of the thyroid gland)
  • Silent (or painless) thyroiditis
  • Amiodarone (medicine to treat some kinds of heart disease)

 

Risks

 

All radiation has possible side effects. The amount of radiation in this test is very small, and there have been no documented side effects. The amount of iodine used is less than the amount in a normal diet.

People with an allergy to iodine in the diet or to shellfish may not be able to have this test. You may be able to have this test if you have an allergy to the iodine in the contrast dye.

Women who are pregnant or breastfeeding should not have this test.

Talk to your provider if you have concerns about this test.

 

Considerations

 

The radioactive iodine leaves your body through your urine. You may need to take special precautions, such as flushing twice after urinating, for 24 to 48 hours after the test. Ask your provider or the radiology/nuclear medicine team performing the scan about taking precautions.

 

 

References

Salvatore D, Davies TF, Schlumberger MJ, et al. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.

Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.

 
  • Radioactivity test

    Radioactivity test - illustration

    Radioactive iodine uptake test is a type of nuclear test performed to evaluate thyroid function. The patient ingests radioactive iodine (I-123 or I-131) capsules or liquid. After a time (usually 6 and 24-hours later), a gamma probe is placed over the thyroid gland to measure the amount of radioactivity in the thyroid gland. The values are then compared.

    Radioactivity test

    illustration

    • Radioactivity test

      Radioactivity test - illustration

      Radioactive iodine uptake test is a type of nuclear test performed to evaluate thyroid function. The patient ingests radioactive iodine (I-123 or I-131) capsules or liquid. After a time (usually 6 and 24-hours later), a gamma probe is placed over the thyroid gland to measure the amount of radioactivity in the thyroid gland. The values are then compared.

      Radioactivity test

      illustration

    A Closer Look

     

    Tests for Radioactive iodine uptake

     

     

    Review Date: 2/3/2016

    Reviewed By: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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