Thyroid Cancer Thyroid Function Testing

Radionuclide scanning provides information not only about the size and shape of the thyroid gland, but also about its function. In addition to I-131, technetium-99m pertechnetate or iodine-123 are other similar radioactive substances that are also used for testing. When used to test thyroid function, only a very small amount of radioactive substance is used so that the thyroid gland is not damaged and normal thyroid function is not affected. Pictures of the thyroid gland are then obtained at varying time periods (hours to days) after the ingestion of these substances (either in pill or liquid form). If the substance is avidly taken up by the thyroid gland, then the thyroid gland is considered to be "hot" or overactive. If these agents are not taken up well then the thyroid is called "cold" or underactive. In the scan pictured above, the red areas are "hot" showing that this is a hyperactive thyroid gland. However, in some cases, the results can be confusing. For example, if you have hyperthyroidism, your thyroid may appear "cold" on scanning if you have thyroiditis or inflammation of your thyroid gland. This is because the thyroid gland is destroyed by inflammation, resulting in the release of an overabundance of thyroid hormone into your bloodstream. But because the thyroid cells are destroyed, the thyroid will not take up the RAI because it is damaged. Thus, the thyroid will appear "cold" on the scan even though you have hyperthyroidism or too much thyroid hormone.

Many physicians routinely order a nuclear scan for their patients with thyroid nodules in order to help them determine if a nodule is cancerous or benign. Hot nodules are almost never cancerous, whereas about 10-20% of cold nodules are malignant. In other words, 80 to 90% of cold nodules are benign. Therefore, this test has limited value in trying to determine the nature of a thyroid nodule.