Radioactive Iodine Uptake (RAIU) and Scan
Diagnosing and treating hyperthyroidism requires more than just knowing whether your thyroid gland is overactive. You also need to know what's causing it to act up. That's why you'll need these other tests to identify the cause behind your hyperthyroidism.
The RAIU and scan, critical tests that are usually done together, measure how much iodine the thyroid gland collects and can help your doctor determine whether your overactive thyroid is the result of Graves' disease, toxic multinodular goiter, or thyroiditis.
Your doctor may suggest fasting for eight hours before the test in order to reduce the amount of iodine in your body. You should also alert your doctor to any medications and supplements you take, since many of these substances may contain iodine and can affect the results of the test.
Is radioactive iodine safe?
Many people get nervous when they hear the term radioactive. But the form of radioactive iodine used in radioactive iodine uptake (RAIU) is highly safe. The dose is very low. Pregnant and nursing women, however, should not undergo an RAIU since the radioactive iodine does cross the placenta, is passed in breast milk, and can affect the baby.
The RAIU test is usually done in the nuclear medicine department of a hospital. The test involves ingesting a pill or liquid that contains small amounts of radioactive iodine-123. After six to 24 hours — and sometimes at several intervals — you will return for a measurement of the radioactivity (uptake) and a picture of your thyroid (scan). The scan will reveal where the iodine is concentrated in the thyroid gland. The scan also reveals which parts of the gland are functioning normally by taking up iodine, and which parts are not.
Here's what your doctor might find:
If the hyperthyroidism is caused by Graves' disease, the uptake and scan will show the thyroid is red or “hot,” indicating higher than normal amounts of iodine in the thyroid gland. The scan will show diffuse uptake of the iodine over the whole thyroid.
If the cause is subacute thyroiditis, the uptake will be low because the excess hormone is coming from stored thyroid hormone and not being made in the thyroid gland.
If the cause is toxic multinodular goiter, the thyroid will appear patchy, with the nodules that are producing excess hormone showing up hot and the rest of the thyroid appearing cold. The overall uptake is elevated but less so than in Graves' disease.
If the cause is a single nodule, the scan and uptake will be hot only where the nodule is located.
If the cause is too much thyroid hormone replacement, the scan and uptake will appear cold since the hormone is coming from medication and not the thyroid itself.
In some cases, the patient is injected with an RAI isotope. Again, you'll be asked to return after six to 24 hours — and sometimes at several different intervals — and asked to lie on a table, with your head stretched back, so that the neck is exposed. A camera is then used to scan your neck for radiation.
In addition to blood work and the RAIU test and scan, your doctor may also order other types of tests to help make a diagnosis. For instance, a computerized tomography (CT) scan may be used to detect a goiter or large nodules. An ultrasound may be performed to find out whether the thyroid is enlarged. Once you're receiving treatment for hyperthyroidism, your doctor may do an ultrasound of the thyroid to see if your thyroid has gotten smaller.
Doing a thyroid scan can help your doctor figure out which parts of the gland are functioning normally and which parts are not. An overactive gland will take up more than normal amounts of iodine, while an underactive gland will take up less. A scan can also reveal the presence of one or more overactive nodules, which will take up more than normal amounts of iodine. In addition, a scan can reveal the presence of a tumor, which would appear cold.
An RAIU test and scan is critical to identifying the cause of your hyperthyroidism. Combined with a physical exam and a discussion of your symptoms, the test can help lead you to the best treatment, too.