Thyroid imaging tests are used to diagnose or monitor thyroid conditions such as hyperthyroidism, thyroid nodules, thyroid cancer, enlarged thyroid gland (goiter) and thyroiditis. These tests can help a physician to determine the most effective treatment approach for a patient’s condition.

Types of thyroid imaging tests include:

• Thyroid scan
• Ultrasound
• CAT scan (computed axial tomography)
• MRI (magnetic resonance imaging)
• Octreotide scan
• PET scan (positron emission tomography)

Preparations for thyroid imaging tests vary depending on the type of test. However, most require little or no preparation. Patients may be asked to avoid eating or drinking for a period of time prior to the procedure. Medication regimens also may need to be altered.

Most thyroid imaging tests are outpatient procedures performed in a clinic or hospital. Each test unfolds differently. In most cases, patients are able to go home shortly after the test is complete.

Usually, a radiologist will examine the results of a thyroid imaging test within a few days and report the findings to the patient’s physician. The physician will then share these findings with the patient. 

Thyroid imaging tests are considered to be generally safe, with few or no harmful side effects. Various treatments are available if a thyroid imaging test confirms the presence of a thyroid problem. In many cases, a physician will prescribe a thyroid medication to treat the disease. In other cases, surgery may be necessary.

Thyroid imaging tests include various procedures that help a physician to see a patient’s thyroid gland. They often are recommended after thyroid blood testing indicates a potential thyroid problem. Thyroid conditions that may be diagnosed or monitored using imaging tests include hyperthyroidism, thyroid nodules, thyroid cancer, enlarged thyroid (goiter) and thyroiditis.

Thyroid scans are among the most commonly used methods of examining thyroid nodules, which are growths of cells in the thyroid that form a cancerous or noncancerous lump. In a thyroid scan, an isotope of radioactive iodine is injected into the patient, usually through a vein inside the elbow or the hand. Thyroid scans can also be performed after use of a capsule that is swallowed. The iodine travels throughout the body, including the thyroid. After the patient lies down, a special camera is used to take images of the gland.

Nodules that create high levels of thyroid hormone take up more of the radioactive isotope and appear clearly on the scan. They are known as “hot” nodules, and they are almost always noncancerous (benign). Nodules that take up an amount of radioactive iodine similar to normal cells are known as “functioning” nodules. These, too, are usually not cancerous and do not require a biopsy.

Nodules that do not create high levels of hormones appear as defects or holes in the scan and are known as “cold” nodules. They may be either benign or malignant (cancerous). Patients diagnosed with these nodules typically are asked to undergo a fine-needle aspiration biopsy to analyze the nodule for signs of cancer.

However, a thyroid scan cannot by itself distinguish whether a nodule is benign or malignant. For this reason, a biopsy typically is used to diagnose cancer, and further thyroid scans may be used to monitor the cancer’s potential spread over time.

A thyroid scan may be performed in conjunction with a radioactive iodine uptake test (RAIU). A patient undergoing an RAIU swallows radioactive iodine or another tracer before being scanned.

Other imaging tests that are used to diagnose thyroid conditions include:

• Ultrasound. High-frequency sound waves are bounced off the patient’s thyroid to create an image of the gland, including its size. This procedure involves no radiation. Ultrasound can help distinguish cysts from solid nodules, but cannot determine whether or not a nodule is cancerous. The procedure may also be used to guide a physician performing a fine-needle aspiration biopsy, in which a needle is guided into a nodule to remove a tissue sample for analysis.
  
  
• CAT scan (computed axial tomography) or MRI (magnetic resonance imaging). These tests may be used to determine the dimensions of a patient’s enlarged thyroid gland (goiter) or to check whether or not cancer has spread. MRI sometimes is sensitive enough to determine whether or not a tumor is benign or malignant.
  
  
• Octreotide scan. This test uses a radioactively tagged hormone to monitor the spread of medullary thyroid cancer, a form of cancer of the thyroid gland arising from the C cell, which secretes the hormone calcitonin.
  
  
• PET scan (positron emission tomography). Radionuclide imaging (nuclear medicine) test that uses glucose combined with a radioactive atom to look for signs of cancer. If cancer cells are present, they will absorb the radioactive tracer.
  
  
• X-ray. Uses low doses of electromagnetic radiation to produce images of body parts on film. X-rays may be ordered to reveal thyroid complications, such as an enlarged heart (cardiomegaly) caused by hypothyroidism.

Preparations for thyroid imaging tests vary depending on the type of test. Most require little or no preparation. Patients may be asked to avoid eating or drinking for a period of time prior to the procedure, or to remain on a low-iodine diet. Medication regimens also may need to be altered.

In preparation for a thyroid imaging test, it may be helpful to ask a healthcare provider about:

• Whether pregnancy or breastfeeding rule out the test
• Restrictions on eating or drinking before the test
• Restrictions on use of nicotine products beforehand
• Wearing loose-fitting, comfortable clothing
• Avoiding carbonated drinks
• Taking medications prior to the test
• Whether recent imaging tests may affect results

Most thyroid imaging tests are outpatient procedures performed in a clinic or hospital. Each test unfolds differently.

During a thyroid scan, the patient has a radioactive isotope injected into a vein. A period of time is then allowed to elapse while the iodine collects in the thyroid gland. This can take between four and six hours. After this time has passed, the patient lies back and stretches the head backward. A special camera is used to take images of the gland, which are then transmitted to computer.

The length of time it takes to complete a thyroid scan depends on how long it takes the isotope to reach the patient’s thyroid gland. A follow-up image sometimes is taken 24 hours later.

Imaging tests such as a CAT scan (computed axial tomography) and MRI (magnetic resonance imaging) involve lying still in special imaging chambers while images of the thyroid gland are obtained. A CAT scan incorporates multiple x-rays to produce three-dimensional images. An MRI produces three-dimensional images through use of powerful magnets. In some cases, contrast mediums may be administered by mouth (orally), through an enema (rectally) or through an injection into a vein. Injection of the dye may produce a warm, flushed feeling for several minutes or a metallic taste in the mouth. Both sensations are normal and disappear quickly.

During an ultrasound, a small amount of water-soluble gel (which acts as a conducer) is placed on the part of the neck where the thyroid gland is located. It may also be placed directly on the transducer, the ultrasound machine's device that emits sound waves. The gel does not harm the skin or stain clothing. A technician uses the transducer to send ultrasound waves through the body. The sound waves sent from the transducer bounce off the thyroid gland, creating an image that is transmitted to a computer.

During a PET scan, a radioactive compound of a mixture of sugar and a small amount of radioactively labeled sugar is administered through an injection. A PET scanner device moves over the patient and images are captured by a device called a photomultiplier–scintillator detector. These images are combined with images from a CAT scan to create detailed images.

Similar to a PET scan, octreotide scans require the patient to take in a radioactive material called octreotide. The thyroid is then scanned with an imaging device called a gamma camera over a period of 24 hours.   

In most cases, patients are able to go home shortly after the thyroid imaging test is complete. Patients who undergo PET scans usually are urged to flush the toilet immediately after using it to avoid exposure to radioactive materials during the first 24 hours after the procedure. Patients also may be asked to refrain from sexual activity and to limit time around children for a period after the test. Patients who receive a contrast medium during CAT scans or MRI scanning also may be urged to drink extra amounts of liquid to flush the material from their system.

Usually a radiologist will examine the results of a thyroid imaging test within a few days and report the findings to the patient’s physician. The physician will then share these findings with the patient.  In some cases other testing may be recommended, such as a biopsy, to confirm or rule out cancer.

Most thyroid imaging tests are considered to be safe, with few or no harmful side effects.

Some patients who have a thyroid scan may experience neck discomfort due to stretching that is required during the scan. Patients who have a thyroid scan are exposed to a small amount of radiation.

Patients who undergo CAT scans are also submitted to radiation exposure. However, most experts agree that the diagnostic benefits of CAT scans outweigh the small risk this radiation poses to the patient.

PET scans are generally safe for the patient. Radiation doses are very small, and radionuclides have a low risk of triggering toxic or allergic reactions. However, patients are advised to inform the physician of all allergies, medical conditions and implanted devices (such as pacemakers) before any imaging procedure.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about thyroid imaging tests:

1. Why are you recommending that I have a thyroid imaging test?
   
   
2. Why do you believe this particular type of test is best for me?
   
   
3. What will you learn from this type of test?
   
   
4. When and where will I have this testing?
   
   
5. How should I prepare for the test?
   
   
6. Will I require anyone to drive me home following the test?
   
   
7. What can I expect during the test? Will I feel anything?
   
   
8. When will I get my results?
   
   
9. Do my results show the need for any additional testing or treatment?
   
   
10. How often will I need to undergo follow-up thyroid imaging tests?