The thyroid is a gland found in the neck. Its main function is to make hormones. Hormones are chemicals which are released into the bloodstream. They act as messengers, affecting cells and tissues in distant parts of your body.
Thyroid hormones affect the body’s metabolic rate and the levels of certain minerals in the blood.
How do thyroid scans and uptake tests work?
Thyroid scans and uptake tests use special chemicals called radionuclides. A radionuclide (sometimes called a radioisotope or isotope) is a chemical which emits a type of radioactivity called gamma rays.
In these tests a tiny amount of radionuclide is put into the body, usually by an injection into a vein. (Sometimes it is swallowed, depending on the test.)
There are different types of radionuclides. Different ones tend to collect or concentrate in different organs or tissues.
So, the radionuclide used depends on which part of the body is to be scanned. To test the thyroid gland, radioactive iodine is used.
This is because the thyroid gland uses iodine to make certain hormones in the body. So, when the radioactive iodine is given, it is quickly taken up by the tissues of the thyroid gland.
Cells which are most active in the target tissue or organ will take up more of the radionuclide. So, active parts of the tissue will emit more gamma rays than less active or inactive parts.
Gamma rays are similar to X-rays and are detected by a device called a gamma camera. The gamma rays which are emitted from inside the body are detected by the gamma camera.
The gamma rays are then converted into an electrical signal and sent to a computer.
The computer builds a picture by converting the different intensities of radioactivity emitted into different colours or shades of grey.
For example, areas of the target organ or tissue which emit lots of gamma rays may be shown as red spots (‘hot spots’) on the picture on the computer monitor.
Areas which emit low levels of gamma rays may be shown as blue (‘cold spots’). Various other colours may be used for in-between levels of gamma rays emitted.
Doctors may use the term ‘thyroid scan’ to mean a test that includes a scan of the thyroid structure and a test of how well the thyroid works.
In a thyroid scan doctors are generally looking at the size, shape and structure of the thyroid gland. A thyroid uptake test checks how well the gland is working.
Both tests are based on the same principles and use the same equipment, just in slightly different ways.
How is the thyroid scan performed?
A thyroid scan is an outpatient procedure usually done in the nuclear medicine section of a hospital. This is usually part of the radiology department.
If you have a thyroid scan, you will be asked to take a drink, or swallow a pill containing the iodine. You will then wait for the iodine to be taken up by the thyroid – usually about 4 hours.
At this time you lie under a scintography camera, and it will take pictures which correlate directly to the amount of iodine taken up by the thyroid.
This is done by counting the intensity and location of the gamma rays emitted by the radioactively labeled iodine.
You then leave the hospital, and return in 24 hours to have a second scan performed in the same manner.
There are no limitations during this 24 hour interval except that you will be asked to take precautions when you urinate.
This is because the radioactive iodine is removed from your body by the urine, and it is safer to have others avoid contact with your urine during this time.
Be sure to let the doctor performing the test know if you have a iodine allergy, or if you have ingested a lot of iodine-containing foods (like sushi) prior to the test.
What happens during thyroid scans and uptake tests?
In a thyroid scan you will either swallow a dose of the radioactive iodine or have an injection of the chemical into a vein. The injection is usually given a short time before the test is due to be done.
The capsule or liquid form is usually given approximately 24 hours before the test is due to begin.
If you are given an injection, it may make you feel slightly warm and flushed.
These feelings should pass fairly quickly.
When it is time to take the images, you will be asked to lie on an examination table with your head tipped back. The gamma camera will then take a series of images, capturing pictures of the thyroid gland from different angles.
You will need to remain still for brief periods of time while the camera is taking pictures.
When the scanning is completed, you may be asked to wait until the images have been checked in case additional images are needed.
Occasionally, more images are needed to get better views of certain areas or structures. This does not necessarily mean there was a problem or that something abnormal was found.
You will not be exposed to more radiation during this process.
A thyroid uptake scan is carried out in a very similar way to the scan described above. The main difference is that the scan may be repeated at different times over a 24-hour period.
This allows doctors to see how the iodine gets taken up by the thyroid over a certain time period.
What should I do to prepare for thyroid scans and uptake tests?
Your local hospital should give you specific information to help you prepare for these tests. As these tests involve a small amount of radiation, pregnant women should not have them.
Let your doctor know if you are, or think you could be, pregnant. You should also let your doctor know if you are breast-feeding.
You may be asked to have some blood tests which help to check the function of the thyroid gland.
You should let your doctor know if you are taking any medications or supplements which have iodine in them. This can include things like heart medications, kelp, seaweed or cough syrups.
Your doctor may also ask you to stop taking certain medications for a few days before the test. It is usually necessary to stop eating for a few hours before these tests. Your hospital should be able to give you advice on this.
What can I expect after thyroid scans and uptake tests?
Thyroid scans and uptake tests do not generally cause any after effects. Through the natural process of radioactive decay, the small amount of radioactive chemical in your body will lose its radioactivity over time.
It may also pass out of your body through your urine or poo (faeces) during the first few hours or days following the test. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly.
If you have contact with children or pregnant women you should let your doctor know. Although the levels of radiation used in the scan are small, they may advise special precautions.
Your hospital should give you more advice on this.
Are there any side-effects or complications from a thyroid radionuclide scan?
The term ‘radioactivity’ may sound alarming. But, the radioactive chemicals used in radionuclide scans are considered to be safe, and they leave the body quickly.
The dose of radiation that your body receives is very small. In many cases, the level of radiation involved is not much different to a series of a few normal X-rays. However:
As with any other types of radiation (such as X-ray), there is a small risk that the gamma rays may affect an unborn child.
So, tell your doctor if you are pregnant or if you may be pregnant.
Rarely, some people have an allergic reaction to the injected chemical.
Theoretically, it is possible to receive an overdose when the chemical is injected. This is very rare.
What is significant about whether a nodule is “hot” or “cold?”
Functioning or “hot” nodules only rarely are from cancer. Nearly all thyroid cancers are nonfunctioning or “cold” nodules. Moreover, even among “cold” nodules, cancer is infrequent (less than 5 percent of cases).
While the thought of taking something “radioactive” is not very appealing, it is important to remember that iodine concentrates only in thyroid tissue, making this test safe and also quite specific for thyroid disorders.
A thyroid scan can provide information on both the structure as well as the function of the thyroid gland.
This information can help you and your physician determine if further investigations, procedures, or medications may be worthwhile.