A thyroid scan is a way for your doctor to see the shape of your thyroid. Your doctor will inject a radioactive isotope into your vein. This isotope then travels through the bloodstream to the thyroid. A camera then takes a photo of your thyroid, which is more visible than usual due to the isotope.
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Honor Society of Nursing (STTI) answered
Jack Merendino, MD, Endocrinology Diabetes & Metabolism, answered
There are several kinds of x-ray or imaging studies that can be used in the evaluation of thyroid disease. The most common is an ultrasound which uses high-frequency sound waves to give a 3 dimensional image. This is useful for checking for the presence of solid thyroid nodules--growths within the thyroid gland--which may be solid or fluid filled (cysts). In most cases a thyroid ultrasound gives the most precise information about the anatomy of the thyroid. To get functional information, the best test is a thyroid scan.
Actually, when we say a "thyroid scan," we usually really mean two separate tests: a thyroid scan and radioactive iodine uptake. These tests are done together and the results are interpreted together. The proper interpretation of a thyroid scan and radioactive iodine uptake is often helpful in the diagnosis of many thyroid disorders.
The thyroid scan gives information functional information in a pictorial sense. The test uses a radioactive material--usually technetium, though iodine can be used---to tell whether the thyroid gland is functioning uniformly or whether some areas are making too much thyroid hormone or not enough. The resulting images give a 2 dimensional picture of how the radioactive material is distributed throughout the thyroid gland. An area that has increased amount of the radioactive material is said to be "hot," and an area with diminished radioactive material is said to be "cold." In most cases this information is important for the evaluation of a thyroid nodule. A "hot nodule" may be the source of increased amounts of thyroid hormone in the blood (hyperthyroidism) while a cold nodule may require a fine needle biopsy to exclude cancer.
The radioactive iodine uptake is a quantitative test. Instead of a picture, a number is generated. A high uptake indicates overall increased metabolic activity in a gland. The most common use for this is to distinguish among causes of hyperthyroidism. For example, Graves' disease, which results from overall increased thyroid activity on the basis of an immune problem, usually results in a very high iodine uptake, while Hashimoto's disease, which sometimes results in hyperthyroidism early on, typically would have a very low value.