A thyroid scan in the conventional sense is a method of looking at how much radioactive tracer (iodine or technetium) the thyroid gland has absorbed from the blood through a special scanner and showing the size, shape, and location of the thyroid gland, as well as areas of hyperfunction and hypofunction.
Another type of thyroid scan is a whole-body scan, for people who have been treated for thyroid cancer, to check if the cancer has metastasized to other parts of the body.
Why do you need a thyroid scan?
- To help doctors find thyroid problems
- To clarify whether thyroid cancer has metastasized to other parts of the body outside the gland. This is when a whole-body scan comes in handy.
Talk to your doctor about these questions before the scan
- If you are taking regular medication, tell your doctor the type and name of the medication and how much you are taking. Thyroid hormones, antithyroid hormone medications, iodine-containing medications or foods (including iodized salt, kelp, cough syrup, multivitamins, and the arrhythmia medicine amiodarone) may affect the results of the thyroid scan. If you are taking them, your doctor will tell you if and when to stop taking them.
- Tell your doctor if you are allergic to elemental iodine. However, the dose of iodine used for the scan is so small that even if you are allergic to iodine, it is rare that you actually have an allergic reaction.
- Tell your doctor if you have had any severe allergic reactions (systemic) from bee stings, eating shellfish, etc.
- Tell your doctor if you have had a follow-up test using iodine or other radioactive substances, such as an enhanced CT, within 1 month, as this can affect the results of an upcoming thyroid scan.
- Women should also tell their doctor if they are pregnant or breastfeeding.
What should I do to prepare for the scan?
Before the thyroid scan, a blood test will be done to measure thyroid hormone levels (TSH, T3, and T4) in the blood. The following preparations can be made before the scan:
- Stop eating a few hours before the test.
- Stop taking certain medications (as prescribed by your doctor).
- Stop taking certain medications (as prescribed).
- If the test is to confirm a diagnosis of thyroid cancer, a period of low iodine diet is also required.
- Take a dose of radioactive iodine orally before the thyroid scan, or inject technetium intravenously. Iodine capsules or iodine solution are odorless and are taken 4 to 24 hours before the scan; technetium will be injected 5 to 30 minutes before the scan. The exact timing and dose depends on which tracer is used.
- Remove dentures, upper body jewelry, and metalwork.
- Learn more from your doctor about the role, purpose, procedure, risks and precautions of the test and then sign the risk informed consent form.
How is the scan performed?
Scans are usually performed in the nuclear medicine department of the hospital’s radiology department.
Both iodine and technetium are used as radiotracers for the scan. The subject can either take iodine orally 4 to 24 hours before the scan; or technetium can be given intravenously 5 to 30 minutes before the scan. The scan can be performed 30 minutes after the tracer is administered, and 24 hours later, one or more additional scans may be required. Each scan takes only a few minutes.
The subject is scanned lying on his or her back, with the head tilted back and the neck straightened and held still. This position may feel uncomfortable, but it is essential to ensure accurate results. The gamma scintillation camera will photograph the thyroid gland from three different angles.
A full-body scan is performed to confirm the diagnosis of thyroid cancer.
Once the scan is complete, the subject can move around normally. However, when urinating, it is important to note that the radiotracer is excreted primarily in the urine and takes about 24 hours to clear. It is important to flush the toilet carefully (2 flushes) and wash hands thoroughly after urinating during this time.
What are the risks of scanning?
The scan and the tracer
The risk of damage to the cells and tissues of the subject from radiation from the scan and tracer is small and usually negligible compared to the benefits of the test.
To avoid radiation exposure to the baby and fetus, this test should not be performed on pregnant women. Caution is also needed for breastfeeding women and children.
Results of the scan
A normal thyroid scan indicates that “the thyroid is a butterfly-shaped gland, approximately 5 cm in length and width, and the radiotracer is evenly distributed within the thyroid.
An abnormal scan shows a thyroid that is larger or smaller than normal and also shows areas of hyperactivity (hot nodules) and hypoactivity (cold nodules) within the gland. Cold nodules are usually associated with thyroid cancer.
After thyroid cancer surgery, a whole-body scan can also show if the cancer has metastasized to other parts of the body.