Thyroid Imaging Tests: MRI, RAI-U, CT, and Ultrasound

by Mary Shomon Patient Advocate

Imaging tests for your thyroid

A variety of medical imaging tests may be used by your doctor to diagnose or manage your thyroid condition. Learn more about the different tests doctors use to visualize the size of your thyroid and evaluate nodules or lumps that may point to a problem.

Doctor is giving young patient an ultrasound

Thyroid ultrasound

Your doctor may request an ultrasound of your thyroid to evaluate nodules, lumps, or the enlarged thyroid condition known as goiter. Ultrasound can determine the size and shape of your gland itself, as well as the characteristics of thyroid nodules or lumps, such as whether they are solid or fluid-filled. Ultrasound cannot, however, determine whether you are hypothyroid or hypothyroid, or differentiate between benign and cancerous thyroid nodules.

Scanning of a thyroid

How a thyroid ultrasound is performed

For a thyroid ultrasound, your doctor will apply some gel to an ultrasound wand, which will then be passed over your neck area so the doctor can make a visual evaluation of your thyroid gland.

Patient undergoing MRI

Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) can provide your doctor with a detailed visualization of your thyroid, including whether it is enlarged or atrophied, and any irregularities in its shape. MRI cannot, however, determine whether you are hypothyroid or hypothyroid, or differentiate between benign and cancerous thyroid nodules. A benefit of MRI is that it requires no exposure to radiation.

Patient in MRI machine

How a thyroid MRI is performed

For a thyroid MRI, you will need to remove any metal jewelry, and metal on your clothing, and advise your doctor in advance of any pacemakers or metal inserts in your body. During the MRI, you will need to remain still in the MRI machine for up to an hour. You can also expect to hear a loud banging noise from the machine. Many MRI testing facilities provide earplugs or headphones.

Doctor operating CT scanner in hospital

Computed tomography (CT/CAT) scan

Doctors sometimes evaluate your thyroid using a computed tomography (CT) scan, which is also sometimes called a “CAT scan.” The CT scan is a type of cross-sectional x-ray that can visualize the size and shape of your thyroid gland, and in some cases detect or evaluate the size and shape of larger thyroid nodules or lumps. A CT scan cannot, however, determine whether you are hypothyroid or hypothyroid, or differentiate between benign and cancerous thyroid nodules.

Preparing for a CT scan

How a CT scan of your thyroid is performed

For a CT scan of your thyroid, you may require intravenous injection of a contrast material prior to the scan. The actual scan is not likely to take more than 30 minutes to an hour.

Human Thyroid Gland

Radioactive iodine uptake (RAI-U)

The radioactive iodine uptake (RAI-U) test is the imaging test that provides the most detailed information about your thyroid. RAI-U is used by physicians to diagnose Graves’ disease, to identify the cause of hyperthyroidism, to more thoroughly evaluate thyroid enlargement, and to determine if your thyroid nodules have characteristics that make them more likely to be cancerous.

Colleagues doctors discussing work

How an RAI-U test is performed

Prior to an RAI-U test, you will likely be asked to refrain from iodine-rich foods and supplements for a specific timeframe, as they can interfere with the test’s accuracy. Up to six hours before the test, you will be given an oral dose of radioactive iodine or radioactive technetium. For the test, a probe is passed over your thyroid area to detect the radioactive iodine.

Male doctor showing record to senior patient

Understanding radioactive iodine uptake (RAI-U) results

In an RAI-U test, how your thyroid—and nodules--absorb the radioactive material can help your doctor evaluate your condition. If your thyroid is overactive–hyperthyroid—it will absorb greater than normal levels. Your doctor will also be able to see if your nodules are absorbing iodine (known as “hot nodules.”) About 15 percent of nodules that don’t absorb iodine (known as “cold nodules”) are cancerous, compared to about 5 percent of hot nodules.

Mother breastfeeding her infant

Thyroid imaging tests, pregnancy, and breastfeeding

CT scans are usually avoided during pregnancy unless the need for the test outweighs the risks. RAI-U is never performed on women during pregnancy. If you are breastfeeding and need an uptake test, you may want to request a scan with technetium instead of radioactive iodine. Technetium has a much shorter half-life in your body. After a technetium scan, your doctor will tell you how long to “pump and dump” your breast milk—usually around three days—before it’s safe to resume breastfeeding.

Mary Shomon
Meet Our Writer
Mary Shomon

Mary Shomon is a patient advocate and New York Times bestselling author who empowers readers with information on thyroid and autoimmune disease, diabetes, weight loss and hormonal health from an integrative perspective. Mary has been a leading force advocating for more effective, patient-centered hormonal healthcare. Mary also co-stars in PBS’ Healthy Hormones TV series. Mary also serves on HealthCentral’s Health Advocates Advisory Board.