Two general purposes guide the imaging of thyroid tissue. The first is to determine the relative function in different regions within the thyroid gland with a special emphasis on the function of nodules compared with that of the rest of the gland. The second is to determine the presence and size of thyroid tissue. In conjunction with the images, it is also possible to determine the overall thyroid function.
The test is done by giving the patient a capsule containing a small amount of radioactivity (Iodine 123). The patient is given the capsule in the morning and then asked to return approximately four to six hours later, and again 24 hours later. The patient should refrain from eating solid foods for at least two hours before and two hours after the capsule is given. The patient should not experience any side effects from the capsule.
When the patient returns later in the day a measurement to determine how much of the radioactivity the thyroid absorbed, as well as images of the thyroid, are taken. The uptake measurement takes approximately five minutes but the images can take anywhere from 15 minutes to 1 hour depending on the amount of radioactivity the thyroid absorbed. The patient also returns the following morning for another uptake measurement that takes approximately 15 minutes.
Before patients are allowed to leave, the images and uptake values are shown to the Radiologist to determine if any additional imaging is necessary.
It is very important that patients are not taking certain drugs or over-the-counter items, such as vitamins that contain iodine. The following list gives the names and medications that will affect the thyroid scan and the length of time they need to be discontinued prior to the thyroid scan.