Radioiodine is used to assess the function of the thyroid gland. A small amount of the substance 2-5mCi (which will not affect the function of the gland) is given either as a pill or a tasteless liquid and images of the gland are taken afterwards.
Normal thyroid glands take this uniformly and a small amount is also taken up by the Salivary glands. If the substance is avidly taken up by the thyroid gland, then whole of the thyroid gland is seen very prominent and salivary glands do not get an opportunity to take up any iodine. This is called overactive thyroid.
A small area may pick up whole of the iodine and rest of the gland does not get a chance because the small area is overactive, this is called cold HOT nodule, a hot nodule is almost never cancer. It may happen that whole of the thyroid picks up iodine other than a small area, this is called COLD nodule. Up to 10% of the cold nodules can be malignant.
Sometimes the results are confusing when in spite of high levels of thyroid hormones, the scan can show decreased uptake and this is called Thyroiditis. We can expect five different kinds of reports – Normal uptake, diffusely increased uptake (Graves’ disease), Hot nodule, Cold nodule and Thyroiditis.