Radioactive iodine uptake Health Article

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

Iodine uptake test; RAIU

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.

The probe is placed 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 stop taking drugs that may interfere with the test.

Drugs that increase results include:

• Barbiturates
• Estrogen
• Lithium
• Phenothiazines
• Thyroid stimulating hormone

Drugs that decrease results include:

• ACTH
• Antihistamines
• Antithyroid drugs
• Corticosteroids
• Lugol's solution
• Nitrates
• Saturated solution of potassium iodide
• Thyroid drugs
• Tolbutamide

Tell your doctor if you have any of these 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

How the test will feel

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

Why the test is performed

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

Normal Values

• 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.