What Is a Thyroid Scan?
A thyroid scan is a procedure that uses nuclear medicine imaging to evaluate the way your thyroid functions. Your thyroid is the gland that controls your body’s metabolism. It is located in your neck. Nuclear medicine uses small amounts of radioactive material to diagnose disease.
There are three different techniques that use nuclear medicine to evaluate your thyroid’s function and structure:
- A thyroid scan produces an image of the thyroid.
- A radioactive iodine uptake test (RAIU) test measures thyroid function without imaging.
- A whole-body thyroid scan, or metastatic survey, is used to evaluate patients with thyroid cancer. The scan evaluates the spread of thyroid cancer to other parts of your body.
For thyroid tests, radioactive iodine typically is used. Your thyroid and most types of thyroid cancer absorb iodine naturally. After the radioactive iodine accumulates in the thyroid tissue, the radioactive emissions can be detected by equipment outside the body, allowing your doctor to evaluate your thyroid function.
Diagnostic Uses of a Thyroid Scan
Thyroid scans can provide valuable information to help your physician determine if your thyroid is working properly.
During these procedures, a radioactive material, called a radioisotope or radionuclide “tracer,” is given by injection, liquid, or tablet. The radioisotope releases gamma rays. A gamma camera or scanner can detect this type of energy from outside the body.
The camera scans the thyroid area. It tracks the radioisotope and measures how it is processed by the thyroid. The camera works with a computer to create images that detail the thyroid’s structure and function based on how it interacts with the tracer.
A thyroid scan can be used to evaluate abnormalities found in a physical exam or laboratory test. The images from this test can be used to diagnose:
- lumps, nodules, or other growths
- an overactive thyroid (hyperthyroidism)
- an underactive thyroid (hypothyroidism)
- goiter (abnormal enlargement of the thyroid)
- thyroid cancer
A thyroid uptake evaluates the function of the gland. When the thyroid absorbs the radioactive iodine, the gland processes it to make thyroid hormones. By measuring the amount of radioactive iodine in your thyroid gland, your physician can evaluate the way you are producing the thyroid hormone.
A thyroid uptake test can provide data for diagnosis of:
A metastatic survey is usually reserved for patients with thyroid cancer. It can determine whether thyroid cancer has spread by detecting where the iodine is absorbed. The procedure is typically performed after thyroid surgery and ablation (removal). It can identify pieces of thyroid that remain after surgery.
Administration of a Thyroid Scan
Any type of thyroid scan is typically performed on an outpatient basis in the nuclear medicine department of a hospital. It can be administered by a nuclear medicine technologist.
Before any type of thyroid scan, you will receive a radionuclide as a pill, liquid, or injection. The amount of time between your receiving the radionuclide and the administration of the test will depend on the type of test you are having:
- If you are having only a scan and you receive an injection, your scan can be done within 30 to 60 minutes of the injection.
- If you receive a radionuclide pill or liquid, you may have to wait from six to 24 hours to allow the iodine to reach your thyroid.
- If you are having both tests together, you will receive the radioactive iodine in liquid or pill form. This will allow you to use the same dose for both tests.
- For a metastatic survey, you will receive radioiodine in pill form. You will have to wait from two to seven days to allow the iodine to travel throughout your entire body.
When you have waited the necessary amount of time for the radioactive iodine to be absorbed, you will return to the nuclear medicine department.
Thyroid Scan Procedure
For a thyroid scan, you will lie down on an examination table. The technologist will tip your head back so that your neck is extended. He or she will then use a scanner or camera to take photos of your thyroid, usually from at least three different angles. You will be required to stay very still while the images are taken. The process takes about 30 minutes.
Radioactive Iodine Uptake Procedure
For an RAIU, you will sit upright in a chair. The technologist will place a probe over your thyroid gland, where it will measure the radioactivity present. This test takes several minutes. Twenty-four hours after the test, you will return to the nuclear medicine department to have another set of readings taken. This allows your physician to determine the amount of thyroid hormone produced between the two exams.
Metastatic Survey Procedure
For a metastatic survey, you will lie down on an exam table. Scans of your body will be taken from the front and the back while you lie very still. For some people, these scans are uncomfortable.
Recovery from a Thyroid Scan
After your thyroid scan, you must contact your physician for instructions on how to resume taking your thyroid medication.
The radioactive iodine in your body is passed out as you urinate. You may be advised to drink extra fluids and empty your bladder often to flush out the residual radionuclide. You may need to be careful to protect others from potential exposure to the material. To do this, your doctor may advise you to flush twice after using the toilet for up to 48 hours after the test.
Typically, you can resume your normal diet and activities immediately after any type of thyroid scan.
Risks of a Thyroid Scan
There is a small but safe amount of radiation contained in the radionuclide used in any type of thyroid scan. Your exposure to radiation will be minimal and within the acceptable ranges for diagnostic exams. There are no long-term complications attributed to nuclear medicine procedures.
Allergic reactions to the radionuclide material are extremely rare. The effects are mild when they occur. If you receive an injection of the radionuclide, you may experience mild pain and redness at the injection site for a short time.
Even though the radiation exposure is minimal and short-term, thyroid scans are not recommended for pregnant or breastfeeding women. If you have had a metastatic scan, your physician may recommend refraining from becoming pregnant or fathering a child for six months after the test.
Preparing for a Thyroid Scan
Tell your physician about any prescription or over-the-counter medications you are taking. Discuss how they should be used before and during the test.
You may have to discontinue thyroid medication from four to six weeks before your scan. Some heart medications and any medicine containing iodine also may require adjustments.
For any thyroid scan, you may be asked to avoid certain foods that contain iodine for about a week before your procedure. Typically, you should not eat:
- dairy products
- iodized salt
- seasonings that contain iodized salt
You also should refrain from using:
- cough syrups
- supplements containing iodine
Other drugs that could affect the results of an uptake test are:
- adrenocorticotropic hormone (ACTH)
- Lugol’s solution (an iodine solution)
You should not have any other imaging tests that use radioactive iodine for six weeks before your thyroid scan. A few days before your procedure, your physician may request a blood test to confirm thyroid function.
You may have to fast for several hours before your exam. Food can affect the accuracy of the uptake measurement.
You will have to remove any jewelry or other metal accessories before the test. These may interfere with the accuracy of the scan.
Results of a Thyroid Scan
The images and results of your thyroid scan will be evaluated by a physician who specializes in nuclear imaging. The results will be sent in a report to your doctor.
In a normal thyroid scan, there are no abnormalities in the size, shape, and location of the thyroid gland. It has an even color. Normal results from a metastatic scan indicate an absence of thyroid tissue and no spread of thyroid cancer.
An abnormal thyroid scan may show a thyroid that is enlarged or out of position, indicating a possible tumor. Abnormal measurements also may show that your thyroid gland collected too much or too little of the radionuclide.
Abnormally high levels of thyroid hormone may indicate:
- early stage of Hashimoto’s thyroiditis (a chronic swelling of the thyroid)
- factitious hyperthyroidism (caused by taking too much thyroid medication)
Abnormally low levels of thyroid hormone may indicate:
- iodine overload
- subacute thyroiditis (inflammation of the thyroid gland caused by a virus)
Abnormal results of a thyroid scan may also indicate:
- colloid nodular goiter (thyroid enlargement due to too little iodine)
- Graves’ disease (a type of hyperthyroidism)
- painless thyroiditis (switching between hyperthyroidism and hypothyroidism)
- toxic nodular goiter (enlargement of a nodule on an existing goiter)
Abnormal results from a metastatic study will show that there are locations where the thyroid cancer has spread. The study will also show where residual thyroid tissue remains after surgical removal or ablation, which destroyed the gland.