What Is a Radioactive Iodine Uptake Test?
A radioactive iodine uptake (RAIU)
is one of two kinds of scans used to diagnose thyroid diseases. The other is
called a thyroid scan. An RAIU shows how well your thyroid is functioning.
A thyroid scan shows the
size, shape, and position of the gland.
may recommend an RAIU scan if you have symptoms of an overactive thyroid, blood
work indicating an overactive thyroid, or an enlarged thyroid gland. The RAIU
can provide valuable information for diagnosis and treatment. In some cases,
your doctor may recommend that you also have a thyroid scan along with an RAIU.
How the RAIU Works
The thyroid is a gland in the neck
that controls the body’s metabolism. It does so by making a hormone called
thyroxine (T4) in response to a pituitary hormone called thyroid-stimulating
hormone (TSH). The thyroid gland absorbs iodine from the body to produce T4.
As part of the
RAIU, you will be given a pill or liquid containing radioactive iodine. The
scan will show how much of this radioactive iodine is absorbed by the thyroid.
This is a measure of how well the thyroid is functioning.
Risks of the Test
The dose of
radiation in this test is small and not associated with any dangerous side
effects. However, as an added precautionary measure, the test is not
recommended for pregnant or breast-feeding women. There are potential risks in
exposing a fetus or baby to radioactive material. Inform your doctor if there
is any chance you may be pregnant or if you are breast-feeding. Your doctor can
use other means, such as blood work and physical exams, to monitor your
doctor if you are allergic to iodine or shellfish (allergies to shellfish may
be due to the iodine found in them). This could interfere with your ability to
take this test.
Preparing for the Test
You may be
asked to stop taking medicines, supplements, or foods (including those
containing iodine, such as iodized salt, seaweed, kelp, and shellfish) that
could interfere with the test. Some prepared foods and takeout meals may be
high in iodized salt. Some vitamins and nutritional supplements also contain
iodine (such as multivitamins and supplements that offer thyroid support). In
addition, foods and over-the-counter medications containing red dye may contain
iodine. You will need to avoid all of these products for a week before your
medications may increase the amount of iodine that the thyroid absorbs. These
solution (iodine-based antiseptic)
solution of potassium iodide
may talk to you about temporarily stopping these medications before your scan.
doctor if you have had other X-rays with iodine-based contrast in the last two
weeks. This can influence the results of your scan. In addition, let your
doctor know if you’ve had diarrhea recently since it may affect your ability to
You will be
asked to fast for eight hours before the test and you may need blood work to
see how your thyroid is working around the time of the scan.
Taking the Test
You will be
given a pill or liquid containing radioactive iodine. It will take time for the
iodine to make its way into your system so that your thyroid can absorb it.
You will be
allowed to eat again in an hour or two after swallowing the radioactive iodine.
However, until the test is over, you will have to follow the same dietary
restrictions you followed in preparation for the test.
You will be
asked to return to the test center at certain intervals (usually six and 24
hours after ingesting the radioactive iodine). At this time, you will be asked
to sit down, and the technician will place a device called a gamma probe over
your thyroid gland (on the outside of your neck). There is no pain. Each scan takes
only about five minutes, although you may be asked to sit for additional images
if the first ones are not clear.
probe measures how much radioactive iodine the thyroid has absorbed at the time
of the scan.
excrete the radioactive iodine in your urine for 24 to 48 hours after the test.
The amount of radioactive iodine used in an RAIU scan is so small that you
won’t need to take any precautions.
What Your Results Mean
The results of
your scan will be analyzed in context with your blood work and the other tests
you have had (including a thyroid scan, if you’ve had one).
has retained less iodine than would be expected if the level of radioactivity
in your thyroid is abnormally low. In general, this means that your thyroid has
become inflamed and is not retaining or producing T4 properly.
is absorbing more iodine than would be expected if the level of radioactivity
in your thyroid is high. This means that you are producing too much T4 and that
your thyroid is overactive. In other words, you have hyperthyroidism. The most
likely causes are an autoimmune disease of the thyroid called either Graves’
disease or hyperactive thyroid nodules. These lumps in the thyroid may grow and
increase the total output of thyroid hormone into the blood). Your doctor will
then discuss with you how to proceed.