What Is a Radioactive Iodine Uptake Test?

A radioactive iodine uptake (RAIU) is one of two kinds of scans used to diagnose thyroid diseases. The other is called a thyroid scan. An RAIU shows how well your thyroid is functioning. A thyroid scan shows the size, shape, and position of the gland.

Your doctor may recommend an RAIU scan if you have symptoms of an overactive thyroid, blood work indicating an overactive thyroid, or an enlarged thyroid gland. The RAIU can provide valuable information for diagnosis and treatment. In some cases, your doctor may recommend that you also have a thyroid scan along with an RAIU.

The thyroid is a gland in the neck that controls the body’s metabolism. It does so by making a hormone called thyroxine (T4) in response to a pituitary hormone called thyroid-stimulating hormone (TSH). The thyroid gland absorbs iodine from the body to produce T4.

As part of the RAIU, you will be given a pill or liquid containing radioactive iodine. The scan will show how much of this radioactive iodine is absorbed by the thyroid. This is a measure of how well the thyroid is functioning.

The dose of radiation in this test is small and not associated with any dangerous side effects. However, as an added precautionary measure, the test is not recommended for pregnant or breast-feeding women. There are potential risks in exposing a fetus or baby to radioactive material. Inform your doctor if there is any chance you may be pregnant or if you are breast-feeding. Your doctor can use other means, such as blood work and physical exams, to monitor your condition.

Tell your doctor if you are allergic to iodine or shellfish (allergies to shellfish may be due to the iodine found in them). This could interfere with your ability to take this test.

You may be asked to stop taking medicines, supplements, or foods (including those containing iodine, such as iodized salt, seaweed, kelp, and shellfish) that could interfere with the test. Some prepared foods and takeout meals may be high in iodized salt. Some vitamins and nutritional supplements also contain iodine (such as multivitamins and supplements that offer thyroid support). In addition, foods and over-the-counter medications containing red dye may contain iodine. You will need to avoid all of these products for a week before your RAIU scan.

Certain medications may increase the amount of iodine that the thyroid absorbs. These include:

  • barbiturates
  • estrogen
  • lithium
  • TSH
  • antihistamines
  • antithyroid drugs
  • corticosteroids
  • Lugol’s solution (iodine-based antiseptic)
  • nitrates
  • saturated solution of potassium iodide
  • thyroid drugs
  • tolbutamide

Your doctor may talk to you about temporarily stopping these medications before your scan.

Tell your doctor if you have had other X-rays with iodine-based contrast in the last two weeks. This can influence the results of your scan. In addition, let your doctor know if you’ve had diarrhea recently since it may affect your ability to absorb iodine.

You will be asked to fast for eight hours before the test and you may need blood work to see how your thyroid is working around the time of the scan.

You will be given a pill or liquid containing radioactive iodine. It will take time for the iodine to make its way into your system so that your thyroid can absorb it.

You will be allowed to eat again in an hour or two after swallowing the radioactive iodine. However, until the test is over, you will have to follow the same dietary restrictions you followed in preparation for the test.

You will be asked to return to the test center at certain intervals (usually six and 24 hours after ingesting the radioactive iodine). At this time, you will be asked to sit down, and the technician will place a device called a gamma probe over your thyroid gland (on the outside of your neck). There is no pain. Each scan takes only about five minutes, although you may be asked to sit for additional images if the first ones are not clear.

The gamma probe measures how much radioactive iodine the thyroid has absorbed at the time of the scan.

You will excrete the radioactive iodine in your urine for 24 to 48 hours after the test. The amount of radioactive iodine used in an RAIU scan is so small that you won’t need to take any precautions.

The results of your scan will be analyzed in context with your blood work and the other tests you have had (including a thyroid scan, if you’ve had one).

Your thyroid has retained less iodine than would be expected if the level of radioactivity in your thyroid is abnormally low. In general, this means that your thyroid has become inflamed and is not retaining or producing T4 properly.

Your thyroid is absorbing more iodine than would be expected if the level of radioactivity in your thyroid is high. This means that you are producing too much T4 and that your thyroid is overactive. In other words, you have hyperthyroidism. The most likely causes are an autoimmune disease of the thyroid called either Graves’ disease or hyperactive thyroid nodules. These lumps in the thyroid may grow and increase the total output of thyroid hormone into the blood). Your doctor will then discuss with you how to proceed.