Radioactive Iodine Uptake Test

What is a Radioactive Iodine Uptake Test?

Definition:
Radioactive Iodine Uptake, or RAIU, is a test of thyroid function. The test measures the amount of radioactive iodine (taken by mouth) that accumulates in the thyroid gland. See also "thyroid scan."

Alternative Names: Iodine uptake test; RAIU

How the test is performed:
RAIU is a type of nuclear test that measures how much radioactive iodine is taken up by the thyroid gland in a given time period. You are asked to ingest (swallow) radioactive iodine (I-123 or I-131) in liquid or capsule form. After a time (usually 6 and 24 hours later), you must return to have the radioactivity measured.

A gamma probe is placed over the thyroid gland in the neck to measure the amount of radioactivity in the thyroid gland. This amount of radioactivity is compared with the original dose of radioactivity and reported as a percent of the original dose.

How to prepare for the test:
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.

How the test will feel:
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.

Why the test is 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%
Note: Some laboratories only measure at 24 hours. There may be some variation in values with dietary iodine ingestion and with laboratory procedural differences.

What abnormal results mean:

Increased (greater than 35% at 24 hours is considered elevated):

• hyperthyroidism
• Hashimoto's thyroiditis (early)
• goiter

Decreased:

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

See also "special considerations".

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 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.

Special considerations:
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)

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, SSKI (saturated solution of potassium iodide), thyroid drugs, anti-thyroid drugs, tolbutamide.