What Is Iodine 131 Used For Thyroid Treatment?

Iodine-131 is a radioactive form of iodine that doctors use to treat overactive thyroid conditions and certain types of thyroid cancer. It works by destroying thyroid cells that absorb iodine, which includes both diseased thyroid tissue and some thyroid cancer cells. This treatment is called radioactive iodine therapy and has been used safely for decades.

What Is Iodine 131 and How Does It Work?

Iodine-131 is a radioactive isotope of the element iodine. The thyroid gland is the only part of the body that actively takes up iodine from the bloodstream. This makes iodine-131 a targeted treatment — it goes directly to thyroid cells, whether they are healthy, overactive, or cancerous.

Once inside the thyroid cell, iodine-131 gives off two types of radiation. Beta radiation travels a short distance and destroys the cell from within. Gamma radiation leaves the body and can be detected by a special camera, which is why doctors can also use iodine-131 for imaging scans.

The half-life of iodine-131 is about 8 days. This means half of the radioactivity is gone after 8 days, and the rest continues to decay over the following weeks. Most of the radiation is delivered to the thyroid in the first few days after treatment.

What Thyroid Conditions Are Treated With Iodine 131?

Doctors most commonly use iodine-131 to treat two broad types of thyroid conditions:

Hyperthyroidism

Hyperthyroidism means the thyroid gland produces too much thyroid hormone. Graves disease is the most common cause. Toxic multinodular goiter and toxic adenoma (a single overactive nodule) are other causes. Iodine-131 destroys enough thyroid tissue to bring hormone production back to normal. Most patients become hypothyroid (too little thyroid hormone) after treatment, which is then managed with daily thyroid hormone medication.

Differentiated Thyroid Cancer

After surgery to remove the thyroid gland (thyroidectomy), doctors may use iodine-131 to destroy any remaining thyroid tissue or microscopic cancer cells. This is called radioactive iodine ablation. It is most effective for papillary and follicular thyroid cancers — the most common types. It is not used for medullary or anaplastic thyroid cancer because those cells do not absorb iodine.

How Is Iodine 131 Given for Thyroid Treatment?

Iodine-131 is given by mouth, usually as a single capsule or liquid. The dose depends on the condition being treated. For hyperthyroidism, the dose is typically lower. For thyroid cancer, the dose is higher.

Before treatment for thyroid cancer, patients follow a low-iodine diet for 1 to 2 weeks. This makes the remaining thyroid cells more eager to take up any iodine, including the radioactive dose. Patients also stop taking thyroid hormone medication temporarily or receive injections of a hormone called recombinant human TSH (Thyrogen) to increase iodine uptake in any remaining tissue.

After taking iodine-131, patients are radioactive for a period of time. Doctors give specific safety instructions to protect others from radiation exposure. These instructions may include sleeping in a separate bed, using a separate bathroom, avoiding close contact with children and pregnant women, and staying home from work for several days.

What Are the Side Effects and Risks of Iodine 131 Therapy?

Most side effects are mild and temporary. They can include neck tenderness or swelling, nausea, dry mouth, altered sense of taste or smell, and a metallic taste in the mouth. These usually resolve within days to weeks.

A common long-term effect is hypothyroidism. For hyperthyroidism, this is expected and is actually the goal in many cases. Patients then take levothyroxine (synthetic thyroid hormone) every day. For most people, this is simple to manage with a daily pill and regular blood tests.

Serious risks are uncommon but real. Long-term studies have found a small increased risk of developing certain secondary cancers, particularly leukemia and cancers of the salivary glands, stomach, and bladder. The absolute risk is low, and for patients with cancer the benefit of treatment usually outweighs this risk.

For men, high doses of iodine-131 may temporarily lower sperm count. For women, high doses may affect menstrual cycles. Current evidence does not show an increased risk of infertility or birth defects from standard treatments, but doctors generally recommend waiting at least 6 to 12 months after treatment before trying to conceive.

How Effective Is Iodine 131 Compared to Other Treatments?

For hyperthyroidism, iodine-131 cures the condition in most patients. The main alternative treatments are antithyroid drugs (methimazole) and surgery. Drugs control hormone levels but do not cure the disease, and relapse is common when they are stopped. Surgery removes the thyroid but requires general anesthesia and leaves a scar. Iodine-131 avoids these limitations, though it nearly always leads to hypothyroidism.

For thyroid cancer, iodine-131 given after surgery significantly reduces the risk of recurrence in patients with higher-risk cancers. For very low-risk cancers, the benefit may be small or none, and many experts now recommend a more selective approach. Clinical guidelines from organizations like the American Thyroid Association help doctors decide who needs radioactive iodine and who can safely avoid it.

Can Iodine 131 Be Used for Other Cancers?

Rarely. Iodine-131 is sometimes used to treat neuroblastoma in children because some of these tumors take up a related substance. But it is not a standard treatment for other cancers. Doctors use other radioactive isotopes for conditions like lymphoma or bone metastases. The unique advantage of iodine-131 for thyroid disease depends entirely on the thyroid gland’s natural ability to absorb iodine.