For this test, a small amount of radioactive iodine is swallowed as a pill or put into a vein. Over time, the iodine is absorbed by the thyroid cells. A special camera is used several hours later to locate the radioactivity.
For a thyroid scan, the camera is placed in front of your neck to measure the amount of radiation in the gland. Abnormal areas of the thyroid that contain less radioactivity than the surrounding tissue are called cold nodules, and areas that take up more radiation are called hot nodules. Hot nodules usually are not cancer, but cold nodules can be either benign or cancer. So this test by itself can't diagnose thyroid cancer.
If a person has thyroid cancer, whole-body radioiodine scans are very useful to look for spread throughout the body. Scans after surgery can also help the doctor see how far a thyroid cancer has spread, if at all.
Radioactive iodine can also be used to treat differentiated thyroid cancer, but it is given in much higher doses. [This type of treatment is described in the section, "Radioactive iodine treatment."]
PET (positron emission tomography) scan: For a PET scan, a kind of radioactive sugar is put into the blood. The amount used is very low. Because cancer cells in the body are growing quickly, they absorb large amounts of the radioactive sugar. A special camera is then use to create a picture of places of radioactivity in the body.
This test can be very useful if your thyroid cancer is one that doesn't take up radioactive iodine. In this case, the PET scan may be able to tell whether the cancer has spread.
Some newer machines are able to do both a PET and CT scan at the same time. This lets the doctor see areas that "light up" on the PET scan in more detail.