Alternate Names: Iodine uptake test; RAIU

How Performed: RAIU is a type of nuclear test of the thyroid (thyroid uptake). You are asked to ingest radioactive iodine (I-123 or I-131) capsules or liquid. After a time (usually 6 and 24-hours later) you must return to have the radioactivity measured. A probe (gamma probe) is placed over the thyroid gland (neck), measures the amount of radioactivity in the thyroid gland, and compares it to the original dose, reporting the result as a percent of the original dose.

How To Prepare: Fast for 8 hours before the test.

Consult with the health care provider if you have a history of factors that may affect the test (see "special considerations"). The health care provider may restrict iodine and thyroid (or anti-thyroid) medications for 1 week before the test.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:

• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

How It Feels: There is no discomfort. You can eat beginning about 1 to 2 hours after ingesting the radioactive iodine, and you can resume a normal diet when the test is finished. For the scanning, you are asked to lie on a table while the scanner is placed over the neck. The scan takes about 30 minutes.

Risks: The risk is minimal. The amount of radioactivity is very small and there have been no documented side effects. However, as with any radiation exposure, this test is not recommended for women who are pregnant or breastfeeding.

The amount of iodine used is less than a normal dietary iodine intake. A history of iodine (contrast dye) allergy does not necessarily contraindicate testing, although history of allergy to dietary iodine (or shellfish) may contraindicate this test.

Why Performed: This test is performed to evaluate thyroid function, particularly when blood tests of thyroid function (for example, T3 or T4 levels) have abnormal results.

Normal Values:

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

Abnormal Results: Increased (greater than 50% at 24-hours is considered significantly elevated):

• hyperthyroidism
• Hashimoto’s thyroiditis (early)
• hypoalbuminemia (low blood albumin levels)
• goiter

• hypothyroidism
• subacute thyroiditis
• iodine overload (excessive iodine ingestion)

• colloid nodular goiter
• Graves’ disease
• painless (silent) thyroiditis
• toxic nodular goiter

Cost:

Special Considerations: If performed with other tests such as thyroid scan and T3RU, this test can help to differentiate primary thyroid disorders (such as Graves’ disease) from secondary disorders (such as tumors).

The radioactive iodine is excreted in the urine. However, the amount of radioactivity is minute, so special precautions may or may not be advised for 24 to 48 hours (often this simply includes flushing twice after urinating). Consult the health care provider or the radiology/nuclear medicine department performing the scan.

Interfering factors:

• iodine-deficient diet
• iodine-excessive diet
• recent (within the past two weeks) radiologic procedures using iodine-based contrast
• diarrhea (may decrease absorption of the radioactive iodine)