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.