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."

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.

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.

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.