Definition of Radioactive iodine uptake:

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

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.