As already discussed previously, thyroid cells take up iodine in order to make the thyroid hormones. This physiological activity is taken advantage of when a radioactive iodine scan is performed. The thyroid cells will take up the iodine and will produce an image of the thyroid on an X-ray film.
If a nodule is not producing thyroid hormones then it will not take up significant amounts of radioactive iodine and will appear 'cold' on the X-ray. A nodule that is actively producing hormone will take up the radioactive iodine and appear darker 'hot' on the X-ray film. Almost all 'hot' nodules are benign. Nodules detected by radioactive iodine scans are classified as cold, hot or warm. 80% of thyroid nodules are cold, 15% are warm, and 5% are hot.