- A thyroid scan is a specialized imaging procedure for examining your thyroid, the gland that controls your metabolism. It’s located in the front part of your neck.
- Typically, the scan works with nuclear medicine to evaluate the way your thyroid functions. Nuclear medicine involves using small amounts of radioactive material to diagnose disease.
- Radioactive iodine is typically used in thyroid tests, including a thyroid scan. Your thyroid and most types of thyroid cancer absorb iodine naturally. The radioactive iodine builds up in your thyroid tissue. A gamma camera or scanner detects the radioactive emissions.
- Your doctor will use the results of this test to evaluate how your thyroid is functioning.
Uses of a thyroid scan
Thyroid scans can help your doctor determine if your thyroid is working properly. You may also have a radioactive iodine uptake (RAIU) test with the scan to measure your thyroid’s reaction.
A radioactive material called a radioisotope, or radionuclide "tracer", is given to you before the test. You may get it through an injection, a liquid, or a tablet. The tracer releases gamma rays when it’s in your body. A gamma camera or scanner can detect this type of energy from outside your body.
The camera scans your thyroid area. It tracks the tracer and measures how your thyroid processes it. 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 (cysts), or other growths.
- Inflammation or swelling.
- An overactive thyroid, or hyperthyroidism.
- An underactive thyroid, or hypothyroidism.
- Goiter, which is an abnormal enlargement of the thyroid.
- Thyroid cancer
An RAIU evaluates the function of the thyroid gland. When your thyroid absorbs the radioactive iodine, it processes the iodine to make thyroid hormones. By measuring the amount of radioactive iodine in your thyroid gland, your doctor can evaluate the way you’re producing thyroid hormones.
A metastatic survey is a type of thyroid scan. It's usually reserved for people 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, or removal. It can identify pieces of the thyroid that remain after surgery.
Thyroid scans are usually performed on an outpatient basis in the nuclear medicine department of a hospital. They can be administered by a nuclear medicine technologist. Your endocrinologist may or may not be there during the procedure.
Before any thyroid scan, you'll receive radionuclide in the form of a pill, liquid, or injection. When you've waited the necessary amount of time for the radioactive iodine to be absorbed, you'll return to the nuclear medicine department.
Thyroid scan procedure
You’ll lie down on an examination table for a thyroid scan without RAIU. The technologist will tip your head back so that your neck is extended. They’ll then use a scanner or camera to take photos of your thyroid, usually from at least three different angles. You’ll be asked to stay very still while the images are taken. The process takes about 30 minutes.
An RAIU is performed 6 to 24 hours after taking the radionuclide. You'll sit upright in a chair for this test. The technologist will place a probe over your thyroid gland, where it will measure the radioactivity present. This test takes several minutes.
You'll return to the nuclear medicine department to have another set of readings taken 24 hours after the first test. This allows your doctor to determine the amount of thyroid hormone produced between the two tests.
Metastatic survey procedure
You’ll receive radioiodine in pill form for a metastatic survey. You’ll need to wait from two to seven days to allow the iodine to travel throughout your entire body.
On the day of the survey, you’ll lie down on an exam table. Scans of your body will be taken from the front and the back while you lie very still. This can be uncomfortable for some people.
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 when you urinate. You may be advised to drink extra fluids and empty your bladder often to flush out the 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.
You can typically resume your normal diet and activities immediately after any thyroid scan.
Risks of a thyroid scan
There's a small but safe amount of radiation contained in the radionuclide used in any thyroid scan. Your exposure to radiation will be minimal and within the acceptable ranges for diagnostic exams. There are no known long-term complications of having a nuclear medicine procedure.
Allergic reactions to the radionuclide material are extremely rare. The effects are mild when they occur. You may experience mild pain and redness at the injection site for a short time if you receive an injection of the radionuclide.
Even though the radiation exposure is minimal and short term, thyroid scans aren’t recommended for pregnant or breastfeeding women. Your doctor may recommend that you avoid becoming pregnant or fathering a child for six months after the test if you’ve had a metastatic scan.
Preparing for a thyroid scan
Tell your physician about any prescription or over-the-counter medications you're 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 shouldn't 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 RAIU are:
- Adrenocorticotropic hormone (ACTH)
- Lugol’s solution, which contains iodine
You shouldn't have any other imaging tests that use radioactive iodine for six weeks before your thyroid scan. A few days before your procedure, your doctor may request a blood test to confirm that your thyroid function is still abnormal. Thyroid scans are used as secondary diagnostic tools to other tests, such as blood work. A scan isn't typically used when thyroid functions are normal. An exception to this is when there are nodules or goiters present.
You may have to fast for several hours before your exam. Food can affect the accuracy of the RAIU measurement.
You'll have to remove any jewelry or other metal accessories before the test. These may interfere with the accuracy of the scan.
What does a thyroid scan do?
- A thyroid scan can provide information on the size and shape of the gland as well as the overall activity of the gland (is the whole thyroid gland overactive or underactive?).
- For example, thyroid scanning is used to determine how active thyroid tissue is in manufacturing thyroid hormone. This feature can determine whether inflammation of the thyroid gland (thyroiditis) is present. It can also detect the presence and degree of overactivity of the gland (hyperthyroidism). In this case the scan reveals increased iodine uptake by the whole gland.
- In addition, a thyroid scan can provide information on specific areas within the thyroid gland and can help to determine whether any localized areas are hyperfunctioning or under functioning compared to the rest of the gland. In these cases spots appear on the image corresponding to areas that take up iodine differently from the rest of the gland.
- Thyroid scanning is especially helpful in evaluating thyroid nodules, particularly after a fine needle aspiration biopsy has failed to provide a diagnosis. A scan will reveal whether a thyroid nodule is "functioning" or "nonfunctioning".
- Doctors refer to hyperfunctioning nodules of the thyroid (those that are actively taking up iodine to produce thyroid hormone more so than surrounding thyroid tissue) as "hot" nodules, and these hyperfunctioning areas are seen on the image of the thyroid gland. A nonfunctioning nodule does not take up iodine and produces a localized "cold" area on the image of the thyroid gland.
- In addition, thyroid scanning can be done to help determine if thyroid cancer has spread beyond the bed of the thyroid in the neck. For this purpose, usually a whole body thyroid scan is performed.