Radioactive iodine therapy: 9 things to know

Radioactive iodine therapy has been used in the treatment of thyroid cancer since the early 1940s. But how does it work? Which patients need it? And how has its use changed over the years?

To learn more about this highly effective and well-established tool, we spoke with Jeena Varghese, M.D., who specializes in endocrine tumors.

What is radioactive iodine therapy?

Radioactive iodine therapy is a radiopharmaceutical. It involves the use of radioactive isotopes — in this case, Iodine-131 (I-131) — to diagnose or treat disease.

How is radioactive iodine therapy used?

Currently, radioactive iodine therapy is only used to treat hyperthyroidism (overactive thyroid gland) and certain types of thyroid cancer. It’s also used to help determine the root causes of hyperthyroidism and to see if certain thyroid cancers have spread to other parts of the body.

How does radioactive iodine therapy work?

The thyroid cell’s job is to make hormones that govern various body processes, such as temperature regulation and metabolism. There are two different types: follicular and para-follicular. Follicular thyroid cells need iodine to do their job, so they take it from the foods we eat. And when someone has a thyroid disorder, we can often use that same mechanism to diagnose and treat it. So, in a sense, radioactive iodine therapy was the first true targeted therapy.

Before administering radioactive iodine therapy, we may have patients follow a low-iodine diet for a few days. This deprives the thyroid of iodine and makes it “hungry” for that element. Then, we give patients radioactive iodine molecules in the form of a pill. Since thyroid cells can’t tell the difference between radioactive and non-radioactive iodine, they absorb it quickly. Any radioactive iodine not absorbed by the thyroid is excreted by the body within a few days.

When used for diagnosis, very small amounts of radioactive iodine help highlight the areas where cancerous thyroid cells are on scans.

When we’re using radioactive iodine therapy to treat thyroid cancer or hyperthyroidism, higher doses are required. The iodine molecules emit radiation, which destroys the malfunctioning thyroid cells.

Can all thyroid cancers be treated using radioactive iodine therapy?

No. It’s only used for patients with certain types of differentiated thyroid carcinomas — specifically, papillary and follicular thyroid cancer. But not even all differentiated carcinomas have the ability to take up iodine. Undifferentiated or medullary thyroid carcinomas will not benefit from iodine therapy.

Can any other organs or cancers be treated with radioactive iodine therapy?

No. Very few cells in the body absorb iodine. So, the unique advantage to using this material is knowing that it will eventually end up in the thyroid gland.

What are common side effects of radioactive iodine therapy?

Any time you’re treated with radioactive material, there’s a certain amount of risk involved. In this case, the iodine targets the thyroid gland, but other glands, such as salivary glands, may also take up some radioiodine. So, patients may develop dry mouth or an altered sense of taste. The latter is actually quite common, but usually temporary.

Papillary thyroid cancer, in particular, is often seen in young women. Exposure to radiation could affect their fertility, so if they’re of child-bearing age, we usually counsel them to avoid conception for at least a year after treatment.

There’s also a risk of radiation exposure to the people around a patient treated with radioactive iodine therapy. So, we warn those patients to avoid pregnant women and children, stay a certain number of feet away from everyone else, and take other precautions, such as washing their own clothing, eating with separate utensils, and not sharing a bed or a bathroom with anyone else until their treatment is complete.

Usually, patients won’t have to isolate themselves for more than about five days. So, many opt to get treatment on a Friday or a long weekend, then go back to work the following week. But everyone who gets radioactive iodine therapy has to follow these guidelines, so if a patient is unable to do so for any reason — whether space constraints or some other issue — we’ll likely admit them to the hospital.

Can you have radioactive iodine therapy more than once, or is it like regular radiation therapy, where it’s considered fairly risky to irradiate the same area twice?

Historically, radioactive iodine therapy has been given to patients more than once, because for a long time, it was the only treatment available for thyroid cancer. We continue to offer more than one treatment in some patients, if the disease recurs or spreads to other parts of the body.

Radioactive iodine therapy is a systemic and targeted treatment. So, if papillary or follicular thyroid cancer has spread to any other part of the body, we can still use this therapy to find and destroy it. It doesn’t matter where those cancerous cells might be.

The risk of developing a secondary malignancy due to radioactive iodine therapy is usually very, very small, but we do keep that in mind before prescribing it. And we wouldn’t do it if we ever thought the risks outweighed the benefits.

How has radioactive iodine therapy evolved over time?

A decade ago, we gave radioactive iodine therapy to everybody — even someone with the smallest papillary carcinoma. But review of the data shows that not all patients need it. We may not need to treat all thyroid cancers, as most grow slowly and patients have excellent outcomes with surgery alone.

So now, we’re leaning more towards giving less radioactive iodine therapy and doing less aggressive surgeries. Particularly if there’s a very low risk of recurrence, we may recommend only a partial thyroidectomy — or removal of a part of someone’s thyroid gland — and not give them radioactive iodine therapy at all.

Are there any other radiopharmaceuticals being used right now in the diagnosis or treatment of other cancers?

Yes. A radiopharmaceutical called Lutathera (Lutetium-177) has been approved by the Food and Drug Administration (FDA) for the treatment of neuroendocrine tumors.

There’s also a drug called Azedra (ultratrace iobenguane-131) that my colleague Camilo Jimenez, M.D. was instrumental in getting approved by the FDA for the treatment of pheochromocytoma. Azedra is currently the only drug approved by the FDA to treat this rare type of adrenal cancer.

A drug called TheraSphere (Yttrium-90) is also being used to treat liver cancer. And a drug called 177Lu-PSMA-617 is being investigated for the treatment of metastatic prostate cancer. It was granted a breakthrough therapy designation by the FDA in June, and is now under priority review.

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