How to Understand Thyroid Imaging Tests

Doctor examining MRI scans
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Diagnosing thyroid disease is a process that can incorporate numerous factors, including clinical evaluation, blood tests, imaging tests, biopsies, and other tests. In this article, you’ll learn more about the imaging tests that are used as part of thyroid disease diagnosis and management.

Radioactive Iodine Uptake (RAI-U)

A radioactive iodine uptake (RAI-U) test can help tell whether a person has Graves' disease, toxic multinodular goiter, or thyroiditis.

In this test, a small dose of radioactive iodine 123 is given in pill form. Several hours later, the amount of iodine in the bloodstream is measured, often accompanied by an x-ray that views how the iodine concentrates in the thyroid.

Intake of high amounts of iodine in your diet can interfere with the test results, so your doctor will typically recommend that you fast more before the test. Find out from your doctor how long you should fast. Be sure you tell your doctor about any medications or supplements you are taking, particularly those that contain iodine, such as multivitamins, kelp, bladderwrack, and seaweed.

Also keep in mind that if you've had medical tests that used iodine contrast dyes, this may skew your RAI-U results for weeks or months, and make the test results less accurate. Be sure to mention any tests using contrast to your doctor before having RAI-U.

An overactive thyroid usually takes up higher amounts of iodine than normal, and that uptake is visible in the x-ray.

A thyroid that takes up iodine is considered "hot," or overactive, as opposed to a "cold" or underactive thyroid.

  • In Graves' disease, RAI-U is elevated, and the entire gland becomes hot.
  • In Hashimoto's thyroiditis, the uptake is usually low, with patchy hot spots in the gland.
  • RAI-U can show when thyroid nodules are hot. If you are hyperthyroid due to a hot nodule, and not Graves' disease, the nodule will show up as hot, and the rest of your thyroid will be cold. Hot nodules may overproduce thyroid hormone but they are rarely cancerous.
  • RAI-U can also show which thyroid nodules are cold — not taking up iodine — and an estimated 10 to 20 percent of cold nodules are cancerous.

Many doctors like to do the RAI-U test, because it's frequently one they can do in their own offices (and charge for) and they can get results quickly, versus sending blood work out to a lab, which requires several days to process. Some practitioners believe that this test is not, however, as accurate — or as safe — at diagnosing Graves' disease as blood tests.

Note: Radioactive iodine 131 (the type of iodine used for ablation of the thyroid and cancer treatment) is not used in this scan. This scan uses radioactive I-123, which is considered better and safer for testing because it has a shorter half life, and gives off a very low level of radiation.

In some cases, technetium 99M is used instead of iodine. The half-life of technetium is six hours. Technetium is sometimes preferred in women who are breastfeeding, because the radioactivity dissipates more quickly, and a nursing mother could get back to nursing her infant more quickly. Because this test involves radioactivity, it is not performed on pregnant women under any circumstances.

CT Scan

A CT scan, known as computed tomography or "cat scan," is a specialized type of x-ray that is sometimes used to evaluate the thyroid.

A CT scan can’t detect smaller nodules but may help detect and diagnose a goiter, or larger thyroid nodules.

Magnetic Resonance Imaging (MRI)

MRI is done when the size and shape of the thyroid need to be evaluated. MRI can't tell how the thyroid is functioning (i.e., it can't diagnose hyperthyroidism or hypothyroidism), but it can detect enlargement. It is sometimes preferable to x-rays or CT scans because it doesn't require any injection of contrast dye, and doesn't require radiation.

Thyroid Ultrasound

Ultrasound of the thyroid is done to evaluate nodules, lumps, and enlargement of your gland. Ultrasound can tell whether a nodule is a fluid-filled cyst or a mass of solid tissue, but it cannot determine if a nodule or lump is malignant.

Because the thyroid typically enlarges in Graves' disease, and the gland typically reduces when responding to antithyroid drug treatment, some practitioners use ultrasound to monitor the success of antithyroid treatment.


Braverman, MD, Lewis E., and Robert D. Utiger, MD. Werner and Ingbar's The Thyroid: A Fundamental and Clinical Text. 9th ed., Philadelphia: Lippincott Williams & Wilkins (LWW), 2005.