Radioactive Iodine Therapy (I-131)

Radioactive iodine therapy (RAI), also called I-131 therapy, is a nuclear-medicine treatment. Thyroid tissue concentrates iodine, allowing I-131 to deliver more radiation there than to most other tissues. However, unbound iodine circulates and is excreted, and organs such as the salivary glands, stomach, bladder, and gonads also receive some exposure.

There are two main settings in which RAI is used:

• Hyperthyroidism (an overactive thyroid, especially from Graves' disease or toxic nodules) — to permanently reduce thyroid activity.
• Selected differentiated thyroid cancers — after risk assessment, to ablate remnant tissue, reduce recurrence risk, or treat iodine-avid disease. Not every patient needs RAI after surgery.

The doses used in these two settings are very different. This guide covers both, but please follow the specific advice given by your treating team for your situation.

Common Indications for Radioactive Iodine Therapy

Your endocrinologist or oncologist may recommend RAI for:

• Graves' disease that has relapsed after antithyroid medication or is unsuitable for surgery.
• Toxic multinodular goitre or toxic adenoma causing hyperthyroidism.
• Thyroid cancer (papillary or follicular type) after total thyroidectomy, to ablate residual thyroid tissue and treat any microscopic remaining cancer.
• Recurrent thyroid cancer that takes up radioactive iodine.

RAI is not used for medullary thyroid cancer or anaplastic thyroid cancer, because those types do not concentrate iodine.

What exactly does the treatment do?

I-131 is swallowed as a capsule or, less commonly, a drink. It is absorbed from the gut into the bloodstream and concentrated by:

• The thyroid gland itself.
• Any thyroid cancer cells that retain the ability to take up iodine.
• The salivary glands and stomach lining to a smaller extent.

The radiation can damage iodine-avid cells over days and weeks. After treatment for hyperthyroidism, hypothyroidism is common and may occur months or years later, so lifelong thyroid-function follow-up is needed. After thyroidectomy for cancer, thyroid hormone is already generally required.

What happens on treatment day?

For most outpatients:

• You arrive at the nuclear medicine department.
• Pregnancy status is assessed and testing is performed when applicable under local policy.
• The nuclear medicine team gives you the I-131 as a capsule (or occasionally a small drink), swallowed with water.
• You are observed for a short period and then discharged with detailed radiation-safety instructions.

Whether treatment is outpatient or requires a protected room depends on the administered activity, national regulations, measured radiation level, home circumstances, and local policy.

Do I need to prepare?

Preparation varies significantly between hyperthyroidism treatment and thyroid cancer treatment.

For hyperthyroidism:

• Antithyroid medicines may need adjustment, but timing depends on the medicine, indication, and risk of worsening hyperthyroidism. Do not stop them without explicit instructions.
• A low-iodine diet or avoidance of iodine-containing medicines and supplements is used in some protocols, not by a universal schedule.
• Tell the team about recent iodinated contrast, amiodarone, supplements, and other iodine exposure because these can reduce uptake for a variable period.
• I-131 therapy is contraindicated in pregnancy. Breastfeeding must have stopped sufficiently in advance, and breastfeeding must not restart for the current child after treatment. The treating service must give the exact interval and plan.

For thyroid cancer:

• The team may use a low-iodine diet and will specify its duration.
• TSH stimulation may involve thyroid-hormone withdrawal or recombinant human TSH; the choice depends on treatment intent and clinical circumstances.
• Follow the department's specific food and drink instructions for the capsule.

For everyone:

• Follow hydration and bladder-emptying advice that accounts for kidney, heart, or fluid-restriction conditions.
• Tell the team about all medications, supplements, and recent imaging.

What are the radiation-safety precautions afterward?

Will the treatment be painful?

Taking the capsule is no different from swallowing any other pill. There are no procedures, no injections during the treatment itself, and no pain at the time.

Possible early effects include:

• Neck soreness or swelling from radiation thyroiditis; ask the team which pain relief is safe for you.
• Dry mouth or altered taste from salivary gland involvement.
• Mild nausea in the first 24 to 48 hours.
• Fatigue for a few days.

Seek urgent advice for marked neck swelling, difficulty breathing or swallowing, persistent vomiting, severe eye symptoms, or symptoms of severe worsening hyperthyroidism.

What happens to thyroid function after the treatment?

In hyperthyroidism, thyroid activity usually changes over weeks to months. Treatment can fail or require further therapy, while hypothyroidism may develop early or much later. The treating clinician sets the blood-test schedule and lifelong follow-up.

In thyroid cancer, RAI does not guarantee removal of all thyroid or cancer cells. Follow-up is risk-adapted and may include thyroid hormone, thyroglobulin and antibody tests, ultrasound, and selected additional imaging.

Are there long-term risks?

The doses used for hyperthyroidism have an excellent long-term safety record. The doses used for thyroid cancer ablation are larger and carry a small long-term risk of:

• Damage to salivary glands causing chronic dry mouth.
• Damage to tear glands causing dry eyes.
• Slightly increased risk of certain second cancers, particularly with very high cumulative doses.
• Effects on fertility in men with high cumulative doses.

Your treating team will weigh these risks against the benefit of preventing cancer recurrence.

What are the important limitations and safety checks?

RAI only treats tissue that takes up iodine and is not appropriate for every hyperthyroid condition or thyroid cancer. Response is not guaranteed, and the appropriate activity and intent must be individualized. Pregnancy is an absolute contraindication; breastfeeding requires advance cessation and must not resume for the current child. Kidney function, blood counts for higher activities, eye disease in Graves' disease, home contacts, fertility goals, and ability to follow precautions all require assessment.

Preparation and radiation precautions are radiopharmaceutical- and protocol-specific. Tell the department before the tracer is given if you are or may be pregnant, are breastfeeding, care for a young child, or recently had another nuclear-medicine test. Do not stop medicines, fast, interrupt breastfeeding, or follow a fixed distancing period based only on a general webpage; obtain written instructions for the exact tracer, activity, and examination from the nuclear-medicine team.

Questions to ask your nuclear-medicine team

• What is the treatment goal, why is I-131 appropriate for me, and what alternatives are reasonable?
• Which medicines, supplements, foods, and recent contrast exposures affect my preparation—and exactly when should each change?
• What pregnancy, breastfeeding, fertility, and conception restrictions apply to me?
• Given my administered activity and household, what exact written distance, sleeping, hygiene, work, travel, and emergency instructions apply?
• What early and long-term follow-up is arranged, and whom should I contact if I become unwell?

Sources and further reading

• RadiologyInfo.org: General nuclear medicine
• RadiologyInfo.org: Preparing for a nuclear-medicine examination
• IAEA: Basics of quality management for nuclear medicine practices

Conclusion

Radioactive iodine can be effective for selected hyperthyroidism and iodine-avid differentiated thyroid cancer, but it is not automatic, risk-free, or guaranteed to work after one treatment. Safe care requires patient-specific preparation, strict pregnancy and breastfeeding safeguards, individualized written radiation precautions, and long-term thyroid follow-up.