A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The test can show how much tracer is absorbed by the thyroid gland and if it is evenly spread in the gland. This helps your doctor know if the thyroid gland is working properly. The radioactive tracer commonly used in this test is iodine.

A radioactive iodine uptake test is done to find problems with how the thyroid gland works, such as hyperthyroidism. An RAIU test may be done at the same time as a thyroid scan.

Why It Is Done

A radioactive iodine uptake (RAIU) test is done to:

• Find the cause of an overactive thyroid gland (hyperthyroidism).
• Plan treatment for hyperthyroidism.

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 RAIU test results.
  • Thyroid hormones
  • Antithyroid medicines
  • Medicines that contain 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 4 weeks before the RAIU test. These other tests may change the results of the RAIU test.
• Are or might be pregnant.
• Are breast-feeding.

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

To prepare for an RAIU test:

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

Your doctor may ask you to eat a low-iodine diet.

For an RAIU, you will swallow a dose of radioactive iodine. Iodine can be taken as a capsule or a fluid 4 to 24 hours before the test. Iodine has little or no taste.

Just before the test, you will remove your dentures (if you wear them) and all jewelry or metal objects from around your neck and upper body.

Before a radioactive iodine uptake (RAIU) test, you need to sign a consent form that says you understand the risks of the test and agree to have it done. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).