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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

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.

What abnormal results mean

Increased levels may be due to:

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

Decreased levels may be due to:

• Hypothyroidism
• Factitious hyperthyroidism
• Iodine overload
• Subacute thyroiditis

Additional conditions under which the test may be performed:

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

What the risks are

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

People 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 mean you can't have this test. Talk to your health care provider.

Special 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 - 48 hours after the test. Ask your health care provider or the radiology/nuclear medicine employee performing the scan.

References

AACE Thyroid Task Force. American Association of Clinical Endocrinologists medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocr Pract. 2002;8(6):457-469.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders; 2007:chap 244.

Larsen PR, Davies TF, Schlumberger MJ, Hay ID. Thyroid Physiology and Diagnostic Evaluation of Patients with Thyroid Disorders. In: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR, eds. Williams Textbook of Endocrinology. 11th ed. Philadelphia, Pa: Saunders Elsevier; 2008:chap 10.