A thyroid scan uses a radioactive tracer and a special camera to measure how much tracer the thyroid gland absorbs from the blood. The tracer can be swallowed or can be injected into a vein. It travels through your body, giving off radiation signals. The camera "sees" the signals and can measure how much tracer the thyroid absorbs from the blood.

A thyroid scan can show the size, shape, and location of the thyroid gland . It can also find areas of the thyroid gland that are overactive or underactive. The camera takes pictures of the thyroid gland from three different angles. The radioactive tracer used in this test is either iodine or technetium.

A radioactive iodine uptake (RAIU) test may also be done to find problems with how the thyroid gland works, such as hyperthyroidism. For more information, see the medical test Radioactive Iodine Uptake Test.

Another type of thyroid scan, a whole-body thyroid scan, may be done for people who have had thyroid cancer that has been treated. The whole-body scan can check to see if cancer has spread to other areas of the body.

Why It Is Done

A thyroid scan is done to:

• Find the cause of an overactive thyroid gland (hyperthyroidism).
• See whether thyroid cancer has spread outside the thyroid gland. A whole-body scan will usually be done for this evaluation.

How To Prepare

Tell your doctor if you:

• Take any medicines regularly. Be sure your doctor knows the names and doses of all your medicines. Your doctor will instruct you if and when you need to stop taking any of the following medicines that can change the thyroid scan test results:
  • Thyroid hormones
  • Antithyroid medicines
  • Medicines that have iodine, such as iodized salt, kelp, cough syrups, multivitamins, or the heart medicine amiodarone (such as Cordarone or Pacerone)
• Are allergic to any medicines, such as iodine. But even if you are allergic to iodine, you will likely be able to have this test because the amount used in the tracer is so small that your chance of an allergic reaction is very low.
• Have ever had a serious allergic reaction (anaphylaxis) from any substance, such as the venom from a bee sting or from eating shellfish.
• Have had any test using radioactive materials or iodine dye (such as a CT scan with contrast) 4 weeks before the thyroid scan. These other tests may change the results of the thyroid scan.
• Are or might be pregnant.
• Are breast-feeding.

Before a thyroid scan, blood tests may be done to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood.

To prepare for a thyroid scan:

• Do not eat for 2 hours before the test.
• Do not take any antithyroid medicine for 5 to 7 days before the test.