The thyroid gland, a small bowtie-shaped gland just below the Adam's apple, regulates the body's metabolism. In a thyroid scan, a small amount of radioactive iodine is put into the patient's body and then detected with a special camera. Although it does not provide a detailed picture, the thyroid scan locates cells in the body that are behaving abnormally, reports the American Cancer Society. The test is used to evaluate thyroid growths or lumps, which are called nodules, and to monitor patients who have had thyroid cancer.
Mechanism
An isotope of radioactive iodine is injected into the vein or the inside of the patient's elbow, reports MayoClinic.com. Alternatively, the patient may swallow the radioactive iodine as a pill. Iodine is taken up by the thyroid gland and thyroid cells anywhere in the body, such as cancer cells, reports the American Cancer Society.
Several hours after the radioactive iodine is taken, the patient lies on a table and a special camera is placed in front of the neck. The camera produces an image of the thyroid on a computer screen. Cells that have taken up the radioactive iodine appear on the scan.
Use in Thyroid Nodules
These scans are used to evaluate nodules in the thyroid, which are solid or fluid-filled lumps within the gland. Hot nodules, which produce too much thyroid hormone and take up more of the radioactive iodine than normal thyroid tissue, show up on the scan. Hot nodules are not cancerous.
A cold nodule is an abnormal area of the thyroid that takes up less radioactive iodine than surrounding tissue. This abnormal area is nonfunctioning and appears as a hole or defect on the scan. About 5 percent of cold nodules are cancerous.
Benefits and Disadvantages
Thyroid scans provide information about the nature and size of the thyroid gland and detect cancerous thyroid cells that have spread to other parts of the body, reports MayoClinic.com. The biggest disadvantage of a thyroid scan is it cannot tell whether a cold thyroid nodule is benign or malignant.
Another small issue is the patient may experience neck discomfort while stretching the neck during the scan. There is also exposure to a small amount of radiation. This test is more invasive and expensive than an ultrasound test, which can detect nodules in thyroid gland but not whether they are hot or cold.
Monitoring of Cancer Patients
A radioiodine scan of the entire body is used to monitor patients who have had thyroid cancer with differentiated cells, which resemble normal cells, reports the American Cancer Society. This test is not useful for medullary thyroid cancer, as this type of cell does not take up iodine.
Whole-body radioiodine scans are used after a biopsy or surgery to determine how far and where thyroid cancer cells have spread.
Sensitivity
Thyroid scans are most effective if high levels of thyroid-stimulating hormone (TSH) are present in the patient's blood, reports the American Cancer Society. Thus, patients may be instructed to stop taking thyroid hormone replacement for a few days or weeks prior to the test, in an effort to cause the body to release more TSH.
The test is more sensitive to thyroid cells that have spread in the body if the thyroid gland is gone, reports MayoClinic.com. Thus, it may be performed frequently in patients whose entire thyroid has been surgically removed.
