What Is the Radioactive Iodine Uptake Test?

A radioactive iodine uptake test is a type of medical procedure used to assess thyroid gland function. The thyroid gland normally takes up iodine in order to make thyroid hormones. This means that measuring the amount of iodine in the thyroid can indicate how much hormone the gland is producing. The patient is given a drink or pill containing radioactive iodine and, after waiting a number of hours for the iodine to reach the thyroid, a radiation-sensing probe is passed over the gland. This measures the thyroid's level of radioactivity, which may be raised in the condition known as Grave's disease, where the gland becomes overactive.

The radioactive iodine uptake test (RAIU test) is a type of nuclear medicine procedure, in which radioactive materials are used. As the amount of radioactive substance used is very small, the risk of complications is low. Even so, the test is not recommended for women who are pregnant or who are breastfeeding their babies.

Before the test, patients are not allowed to eat for around eight hours. In the week leading up to the test, it may also be necessary to avoid any drugs or foods which could affect the results. These include thyroid hormone medication and those foods which contain high levels of iodine, such as saltwater fish. A diet deficient in iodine could also give a misleading test result, and patients with diarrhea might have problems absorbing the radioactive iodine.

At the start of the testing procedure, patients swallow the radioactive iodine and wait for a period of six to 24 hours before returning to have the thyroid's radioactivity assessed. During the radioactive iodine uptake test, which only takes a few minutes, the patient sits upright in a chair while a probe is positioned several inches in front of the thyroid gland. The probe is sensitive to gamma radiation and allows the amount of radioactivity in the thyroid to be measured. In some cases, two measurements are made at six and 24 hours, while others only take a reading at 24 hours.

Following a radioactive iodine uptake test, the radioactive iodine leaves the body gradually in urine and feces. Patients are advised to drink lots of fluids to help flush the radioactive substance from the body. It is important to wash the hands carefully after urinating and to flush toilets twice. After a couple of days, most of the radioactive iodine will have been lost from the body or will have decayed, and these precautions are no longer necessary.