Radioactive iodine uptake

Definition

Radioactive iodine uptake (RAIU) is a test of thyroid function. It measures how much radioactive iodine is taken up by the thyroid gland in a given time period.

See also:Thyroid scan

Alternative Names

Why the Test is Performed

This test is done to evaluate thyroid function, particularly when blood tests of thyroid function (such as T3 or T4) show abnormal results.

How the Test is Performed

You are asked to swallow a liquid or capsule containing radioactive iodine.

After a certain period of time (usually 6 and 24 hours later), you must return to the testing center so that the amount of radioactivity in the thyroid gland can be measured. This is done using a device called a gamma probe, which is positioned over your thyroid gland along the outside of your neck. You will be asked to lie on a table while the scanner moves over your neck.

The scan takes about 30 minutes.

How to Prepare for the Test

Do not eat for 8 hours before the test.

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test.

Drugs that increase results include barbiturates, estrogen, lithium, phenothiazines, and thyroid stimulating hormone.

Drugs that decrease results include ACTH, antihistamines, corticosteroids, Lugol's solution, nitrates, saturated solution of potassium iodide, thyroid drugs, antithyroid drugs, and tolbutamide.

Tell your doctor if you have other interfering factors:

• Diarrhea (may decrease absorption of the radioactive iodine)
• Recent x-ray test using iodine-based contrast (within the past 2 weeks)
• Too little or too much iodine in your diet Iodine-deficient diet

How the Test Will Feel

There is no discomfort. You can eat beginning about 1 to 2 hours after swallowing the radioactive iodine, and may resume a normal diet when the test is finished.

Risks

The amount of radioactivity is very small and there have been no documented side effects. The amount of iodine used is less than a normal dietary iodine intake. However, as with any radiation exposure, this test is not recommended for women who are pregnant or breastfeeding.

Persons with a history of allergy to dietary iodine or shellfish may not be able to have this test. A history of allergy to iodine (contrast dye) does not necessaily does not mean you can't have this test. Talk to your health care provider.

Considerations

The radioactive iodine leaves your body through your urine. Special precautions, such as flushing twice after urinating, may or may not be advised for 24 to 48 hours. Ask your health care provider or the radiology/nuclear medicine employee performing the scan.

Normal Results

• 6 hours: 3 - 16%
• 24 hours: 8 - 25%

Note: Some laboratories only measure at 24 hours. Values may vary depending on the amount of iodine in your diet. Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Increased levels may be due to:

• Hashimoto's thyroiditis (early)
• Hyperthyroidism
• Goiter

Decreased levels may be due to:

• Hypothyroidism
• Iodine overload
• Subacute thyroiditis

Additional conditions under which the test may be performed:

• Colloid nodular goiter
• Graves disease
• Painless (silent) thyroiditis
• Toxic nodular goiter