Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
 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.
Review Date: 1/18/2006
Reviewed By: Nikheel S. Kolatkar, MD, Clinical and Research Fellow, Division of Endocrinology,
Diabetes, and Hypertension, Brigham and Women's Hospital, Harvard Medical School, Boston,
MA. Review provided by VeriMed Healthcare Network.
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