Test Overview

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. The RAIU test often is done along with a thyroid scan, which shows if the tracer 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.

radioactive iodine uptake test is done to find problems with how the thyroid
gland works, such as

Why It Is Done

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

  • Find the cause of an overactive thyroid gland
  • Plan treatment for hyperthyroidism.
  • Plan treatment for patients who have had thyroid cancer surgery.

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
    • Antithyroid
    • Medicines or supplements that contain iodine, such as iodized salt,
      kelp, cough syrups, multivitamins, or the heart medicine amiodarone (such as Cordarone or
  • 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
  • Have had any test using
    radioactive materials or iodine dye, such as a CT scan, 4 weeks before the RAIU test. These other
    tests may change the results of the RAIU test.
  • Are or might be
  • Are breastfeeding.

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:

  • Follow your doctor’s instructions about not eating before the test. Most often, you will need to stop eating 8 hours before the
  • Tell your doctor about all of the medicines, vitamins, and supplements you are taking. You may need to stop taking some medicines or supplements for a while 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.

You will be asked 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
medical test information form (What is a PDF document?).

How It Is Done

A radioactive iodine uptake (RAIU) test
is done in the nuclear medicine section of a hospital’s radiology department by
a person trained in nuclear medicine (nuclear medicine technologist).

For this test, you will lie on your back with your head tipped backward
and your neck extended. It is important to lie still during this test. A
special machine is placed over your thyroid gland to measure the amount of
tracer absorbed by the thyroid gland. This is not an X-ray machine-it is a scanner that detects the radiation given
off by the tracer. This test takes about 10 minutes and is done 3 to 6 hours after you are given the tracer. Another scan may be done in 24 hours.

After an RAIU test, you can do your regular
activities. But you will be asked to take special precautions when you urinate.
This is because your body gets rid of the radioactive tracer through your
urine. This takes about 24 hours. During this time, it is important to flush the toilet twice each time you use it and wash
your hands thoroughly after each time you urinate.

How It Feels

You may find it uncomfortable to lie
still with your head tipped backward.


There is a slight chance of damage to
cells or tissue from radiation, including the low levels of radiation used for
this test. But the chance of damage from the radiation is usually very low
compared with the benefits of the test.

This test is not done for
pregnant women because of the chance of exposing the baby (fetus) to radiation. This test is also not recommended
for breastfeeding women or young children.


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 radioactive tracer used in this test
is iodine. An RAIU test is done to check for thyroid gland problems, such as

Radioactive thyroid scan and radioactive iodine uptake test (RAIU)

The amount of radioactive tracer in the
thyroid gland is normal. An RAIU test measures the amount of tracer taken up by
the thyroid gland at certain times after the tracer is given. The measured
amount of radioactive tracer in the thyroid gland at each one of these times is
at normal levels.


The test shows either more or less uptake
of tracer than normal in the thyroid gland. If hyperthyroidism is present, abnormal test results may mean certain
conditions are present.

  • A low uptake of tracer by the thyroid
    gland may mean that hyperthyroidism is caused by inflammation of the thyroid
    gland (thyroiditis), taking too much thyroid medicine, or
    another rare condition.
  • A high uptake of tracer spread evenly in
    the thyroid gland may mean that hyperthyroidism is caused by conditions such as
    Graves’ disease.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Taking thyroid medicine.
  • Eating
    foods with iodine, such as shellfish, iodized salt, or kelp.
  • Having other tests using
    contrast materials in the past 4 weeks.

What To Think About


Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.


ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Specialist Medical Reviewer Alan C. Dalkin, MD – Endocrinology

Current as ofMay 3, 2017