Q. Can you please explain the difference between “hot” and “cold” thyroid nodules?

A. “Hot” and “cold” nodules are terms used to describe findings on a a radioactive iodine uptake scan. In this test, the patient is given a small amount of radioactive iodine, usually in pill form, and then the thyroid is scanned using a scintillation counter to see how much of that iodine is taken up by the thyroid. A normal thyroid gland will show diffuse, even uptake of the radioactive iodine (check out the image above). Areas of the thyroid that are very active show more uptake; areas that are less active show less uptake.

You can use both the location and intensity of the uptake to help you figure out what is going on in the thyroid. If there is diffuse increased uptake, it means the entire gland is overactive, and the patient probably has a disease like Graves disease. If there is diffuse decreased uptake, it means the entire gland is underactive, and the patient probably has a disease like Hashimoto thyroiditis. If there are focal areas of increased uptake and focal areas of decreased uptake, it means different parts of the gland are acting differently, and the patient probably has a multinodular goiter.

Sometimes, you’ll just see a single area on the scan – a nodule – that is different than the surrounding thyroid. Some nodules show increased uptake compared to the rest of the thyroid; these are called “hot” nodules. Other nodules take up less iodine than the rest of the thyroid; these are called “cold” nodules. Hot nodules are busy making a lot of thyroid hormone; cold nodules are just sitting there doing nothing.

Although most thyroid cancers present as solitary nodules, so do a lot of other thyroid diseases (like adenomas and even non-neoplastic diseases, like goiter). Most thyroid nodules, in fact, turn out not to be malignant. Still, because a small number do turn out to be malignant, when a patient shows up with a thyroid nodule, you have to investigate it.

One way to do this is with the radioactive iodine scanning that we’ve been talking about here. If you scan the patient’s thyroid, and the nodule turns out to be hot, chances are very good that the nodule is benign (either an adenoma or a dominant nodule in a multinodular goiter). Hot nodules are virtually never malignant. If the nodule is cold, chances are still good that it’s not malignant – but not quite as good: about 10% of cold nodules turn out to be malignant.

The bottom line is that all this scan will tell you is likelihood. To get a definitive diagnosis, you need to get a piece of tissue, either a fine needle aspiration or a needle biopsy.