Test Overview
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.
Your doctor may ask you to eat a low-iodine diet for several days if this test is being done to check for thyroid cancer.
For a thyroid scan, you will either swallow a dose of radioactive iodine
or be given technetium in a vein (intravenously)
in your arm. When and how you take the radioactive tracer depends on which
tracer is used.
- Iodine can be taken as a capsule or a fluid 24
hours before the test. Iodine has little or no taste.
- Technetium is
given 2 hours before the test.
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 thyroid scan, you need to sign a consent form that
says you understand the risks of the thyroid scan 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?).
How It Is Done
A thyroid scan is done in the nuclear
medicine section of a hospital's radiology department by a person trained in
nuclear medicine (nuclear medicine technologist).
The tracer used in this test is either radioactive iodine or technetium. You will either swallow a dose of iodine 4 to 24 hours before the scan or be given technetium in a vein (intravenously) in your arm 15 to 30 minutes before the scan.
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 camera
(called a gamma scintillation camera) takes pictures of your thyroid gland from
three different angles. The test takes
about 30 minutes.
For a whole body thyroid cancer scan, the camera will scan your body from head to toes.
After a
thyroid scan, 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. It is
important to flush the toilet 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.
Risks
There is always 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 breast-feeding women or young children.
Results
A thyroid scan uses a
radioactive tracer and a special camera to make a
picture of the
thyroid gland. The radioactive tracer used in this test is usually iodine or
technetium. A thyroid scan is done to help find problems with the
thyroid gland.
Thyroid scan| Normal: | A normal thyroid scan shows a small
butterfly-shaped thyroid gland about 2 in. (5 cm) long and
2 in. (5 cm) wide with an
even spread of radioactive tracer in the gland. |
|---|
|
Abnormal: | An abnormal thyroid scan shows a thyroid
gland that is smaller or larger than normal. It can also show areas in the
thyroid gland where the activity is less than normal (cold
nodules) or more than normal (hot nodules). Cold
nodules may be related to
thyroid cancer. |
|---|
A whole-body scan will show whether iodine
is in bone or other tissue (iodine uptake) after the thyroid gland has been
removed for cancer. The whole-body scan can check to see if cancer has spread
to other areas of the body.
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
- Blood tests may be done before a thyroid scan
to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood. For
more information, see the topics
Thyroid-Stimulating Hormone (TSH) and
Thyroid Hormone Tests.
- Cancer of
the thyroid is usually treated with surgery. If the tumor is large, has spread
outside the thyroid gland, or has recurred, it may then be treated with very
high doses of radioactive iodine. After treating recurring thyroid cancer, a
scan of the entire body can be done to see where the cancer has spread.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
Credits
| By | Healthwise Staff |
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| Primary Medical Reviewer | E. Gregory Thompson, MD - Internal Medicine |
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| Specialist Medical Reviewer | Matthew I. Kim, MD - Endocrinology |
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| Last Revised | May 31, 2011 |
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