Thyroid Scan
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What is the
test?
There are two types of thyroid nuclear medicine
tests. Both assess the health of your thyroid,
a gland in your neck that controls metabolism.
A thyroid scan produces a picture of the gland
to help evaluate any lumps or inflammation, or
to investigate the cause of an overactive thyroid.
A radioactive iodine uptake test is performed
to see if your thyroid is functioning normally
and to determine why thyroid hormone levels may
be elevated. For both types of test, a small
amount of a weakly radioactive substance, known
as a radionuclide, is either injected into a
vein or given to you as a pill
A thyroid scan is usually ordered when a physical
examination or laboratory finding suggests that
the thyroid is enlarged. If laboratory tests
show an overactive thyroid, a radioactive iodine
uptake test may be ordered at the same time.
A radioactive iodine uptake test measures the
amount of radioactivity in your thyroid after
you've been given a relatively small dose of
radioactive iodine in pill form. Your thyroid
gland absorbs iodine and uses it to make hormones.
Therefore, the amount of radioactive iodine detected
in your thyroid gland corresponds with the amount
of hormone your thyroid is producing.
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How
do I prepare for the test?
If there is any chance that you are pregnant,
or if you are breast-feeding, let your doctor
know. Radionuclides could harm your nursing baby
or a developing fetus, so your doctor will use
an alternative method to diagnose the problem,
such as ordering additional blood tests or monitoring
your symptoms over several weeks to see if they
resolve.
For about a week before a thyroid scan, your
doctor may ask you to avoid certain foods and
medicines that can interfere with the results,
including thyroid hormones and shellfish (which
contain iodine). You might have to fast entirely
for several hours beforehand if you'll be given
a radioactive iodine pill for the test.You might
also need to have blood tests that check thyroid
function.
The preparation for a radioactive iodine uptake
test is almost the same as for a thyroid scan.
However, because you are given radioactive iodine
in pill form for an uptake test, you need to
wait four to six hours, and possibly as long
as a day, before having the scan. (This gives
the radioactive iodine time to reach your thyroid.)
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What
happens when the test is performed?
A radionuclide is either injected into a vein
or given to you as a pill. Timing of the test
then depends on the type of radionuclide your
doctor uses, and whether you will also have an
uptake test. If you are having only a thyroid
scan and your doctor prefers to give a radionuclide
by intravenous injection, the scan can be done
within 30-60 minutes. If you are given radioactive
iodine in pill form, you need to wait four to
six hours, and possibly as long as a day, before
having the scan. (This gives the radioactive
iodine time to reach your thyroid.) If you're
having both a scan and the uptake test, you are
likely to receive radioactive iodine in pill
form. This allows one radionuclide to be used
for both the scan and uptake test, instead of
two, and eliminates the need for an injection.
After you've received the radionuclide and have
waited the appropriate amount of time, a technician
places a radioactivity detector-a camera specially
designed to take pictures of radioactive objects
- against your neck and takes several images.
The camera itself doesn't expose you to any radiation.
This portion of the test usually takes about
half an hour.
An uptake test only takes several minutes and
is performed while you are sitting up. Using
a device that resembles a Geiger counter, the
doctor or technician places a probe several inches
in front of your neck, where the thyroid gland
is located, and measures the percentage of radioactivity
that is retained by the thyroid gland. You return
the next day for follow-up testing to obtain
a second set of uptake readings, which are then
compared with the first set to determine how
much hormone has been formed and secreted in
the interim.
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What
risks are there from the test?
The risk from both varieties of thyroid scan
is minimal. The amount of radioactivity you are
exposed to is comparable to that from a routine
x-ray. The amount of radionuclide used is so
small that it's unlikely to cause side effects
or allergic reactions.
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Must
I do anything special after the test is over?
No. The vast majority of the weakly radioactive
substances used in these tests are cleared from
your body within a day or two. But even before
then, you can interact normally with other people
because there's no risk of exposing them to significant
amounts of radiation from your body.
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How
long is it before the result of the test is
known?
For the thyroid scan, it takes an hour or more
for the pictures to be developed and additional
time for a radiologist to examine them. Your
doctor will probably receive a report within
a day or two. The scan will show the outline,
shape, and position of your thyroid so that the
doctor can determine whether it is enlarged and
whether there are any suspicious growths or nodules.
The scan also provides a rough measure of thyroid
activity, although this has to be confirmed with
a radioactive iodine uptake test.
Uptake test results are available immediately,
but because the initial and follow-up readings
must be compared, it may take your doctor a day
or two to get back to you. To obtain results,
your doctor determines an uptake value, which
is the net result of how much iodine is picked
up by the thyroid, how much is converted to hormone
since the time of administration, and how much
is either leaked or secreted into the bloodstream.
(The thyroid normally secretes hormone in an
orderly fashion based on physical needs; leakage
is less controlled and indicates that the gland
is damaged.) A low reading of radioactivity suggests
that your thyroid gland has retained only a small
amount of iodine. This generally indicates that
the thyroid gland is not producing excess thyroid
hormone, but has become inflamed and is unable
to properly store the hormone, which then leaks
into the bloodstream. A high reading suggests
that your thyroid is overactive, producing an
excessive amount of thyroid hormone.
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