What Imaging Tests Are Used to Diagnose Thyroid Disease?

Doctor examining MRI scans
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Diagnosing thyroid disease is a process that may involve several steps, including a clinical evaluation, blood tests, imaging tests, and biopsies.

In this article, you’ll learn more about the imaging tests that are used as part of thyroid disease diagnosis.

Radioactive Iodine Uptake (RAI-U)

A radioactive iodine uptake (RAI-U) test may be used to diagnose certain types of thyroid disease, like hyperthyroidism.

Since the thyroid gland is the only tissue in your body that can take up iodine, in a RAI-U test, a small dose of radioactive iodine 123 (called I-123) is given to you in pill or liquid form. This radioactive iodine 123 emits radiation, which can be detected through pictures taken by a camera placed by your neck. The pictures reveal how the iodine is concentrated in the thyroid. 

It's important to mention that this form of iodine (I-123) is not harmful to your thyroid cells. However, this test should not be performed in women who are pregnant or breastfeeding.

Additionally, radioactive iodine 131 (the type of iodine used for cancer diagnosis and ablation of the thyroid) is not used to diagnosis the cause of your hyperthyroidism. Radioactive iodine 131 is used to diagnose thyroid cancer. It's also used treat thyroid cancer but in much higher doses.

The results of your RAI-U test will help your doctor make a diagnosis.

Generally speaking, a thyroid that takes up iodine is considered "hot," or overactive. For instance, in Graves' disease, the uptake of iodine is high, so the entire gland is "hot." 

If you are hyperthyroid due to a hot nodule (a nodule that is producing excess thyroid hormone) or multiple nodules (called toxic multinodular goiter), the iodine uptake in that nodule(s) will be high.

On the other hand, the uptake is very low (zero to 2 percent) in people with thyroiditis (inflammation of the thyroid gland). 

Thyroid Ultrasound

Ultrasound of the thyroid is done to evaluate nodules in the thyroid, including small ones that cannot be felt during a physical examination. Using high-frequency sound waves, the ultrasound can help a doctor determine the size of the nodule, as well as whether a nodule is a fluid-filled cyst or a mass of solid tissue. Sometimes periodic thyroid ultrasounds are used to monitor nodules, like to see whether or not they are growing over time and/or developing suspicious features for cancer.

Another way in which a thyroid ultrasound may be used is if your doctor wants to take a sample of a nodule (called a fine needle biopsy). Your doctor will use the ultrasound to guide where the needle goes. 

Lastly, a thyroid ultrasound may be used as an alternative to a radioactive iodine scan if a woman is pregnant or breastfeeding.

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, as well as 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 (in other words, it can't diagnose hyperthyroidism or hypothyroidism), but it can detect thyroid enlargement. MRI is sometimes preferable to a CT scan because it doesn't require any injection of contrast, which contains iodine and can interfere with a radioactive iodine scan.  

A Word From Verywell

In the end, the first imaging test used to examine the thyroid gland is usually an ultrasound. From there, your doctor may move forward with other imaging tests, based on the results of your ultrasound and blood tests. Of course, if you have any questions about how your thyroid is being imaged, please do not hesitate to ask your doctor. Remain proactive in your thyroid knowledge and health. 

Sources:

American Cancer Society. (n.d.). Tests for Thyroid Cancer. 

American Thyroid Association. (n.d.). Thyroid Nodules.

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.

Kravets I. Hyperthyroidism: Diagnosis and Treatment. Am Fam Physician. 2016 Mar 1;93(5):363-70.