Radioactive iodine therapy (RAI), also called I-131 therapy, is one of the few examples of a nuclear-medicine technique used to treat disease rather than just diagnose it. The thyroid gland naturally concentrates iodine from the bloodstream — and when that iodine is mildly radioactive (I-131), it delivers a targeted dose of radiation to the thyroid tissue while leaving the rest of the body largely unaffected.
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.
- Thyroid cancer (after thyroid surgery) — to destroy any remaining thyroid tissue or thyroid cancer cells.
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 given off by I-131 destroys nearby cells over the following days and weeks. Thyroid activity gradually drops, often resulting in permanent hypothyroidism (an underactive thyroid) that needs lifelong thyroid hormone replacement — this is an expected outcome, not a complication.
What happens on treatment day?
For most outpatients:
- You arrive at the nuclear medicine department.
- A urine pregnancy test is done if you are a woman of reproductive age.
- 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.
For higher-dose treatment (typically thyroid cancer ablation), you may need to be admitted to a specially shielded isolation room for 1 to 3 days until radiation levels drop sufficiently for safe discharge.
Do I need to prepare?
Preparation varies significantly between hyperthyroidism treatment and thyroid cancer treatment.
For hyperthyroidism:
- You may need to stop antithyroid medications (carbimazole, methimazole, propylthiouracil) for several days before treatment.
- You may be asked to avoid iodine-rich foods (seaweed, kelp supplements, large amounts of seafood) and iodine-containing medications for 1 to 2 weeks before treatment.
- Tell the team about any iodine-based scans (CT with contrast) in the previous 4 to 6 weeks — they can interfere with the treatment.
- Pregnancy must be ruled out before treatment. Breastfeeding must be stopped.
For thyroid cancer:
- A more rigorous low-iodine diet for 1 to 2 weeks before treatment.
- Either stopping thyroid hormone replacement for several weeks (to raise TSH) or using a TSH-stimulating injection (rhTSH, Thyrogen) shortly before treatment — your team will explain which approach applies.
- Fasting for several hours before swallowing the capsule.
For everyone:
- Hydrate well in the 24 to 48 hours before treatment.
- Empty your bladder frequently after treatment to help excrete unbound iodine.
- Tell the team about all medications, supplements, and recent imaging.
What are the radiation-safety precautions afterward?
Radiation safety after RAI is real and time-limited
For a period after treatment (typically 1 to 4 weeks, depending on dose), you need to take precautions to limit radiation exposure to others:
- Sleep alone for several days to a week.
- Avoid close prolonged contact with young children and pregnant women — generally meaning no shared cuddling on a couch, no carrying a small child on your hip, no sitting close on long car rides.
- Use separate utensils and toiletries for the first few days.
- Flush the toilet twice after each use, and wash your hands well.
- Drink plenty of water and suck on sour sweets or chew gum in the first 2 days to encourage saliva flow and reduce salivary gland exposure.
- Avoid pregnancy for 6 to 12 months after treatment (women) or for several months (men).
- Avoid breastfeeding for at least the rest of the current breastfeeding episode — RAI is concentrated in breast milk and can damage an infant's thyroid.
Your nuclear medicine team will give you specific written instructions tailored to your dose and family situation.
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.
Common side effects in the days afterward include:
- Mild neck soreness (radiation thyroiditis) — usually settles with paracetamol.
- Dry mouth or altered taste from salivary gland involvement.
- Mild nausea in the first 24 to 48 hours.
- Fatigue for a few days.
Serious side effects are uncommon at typical doses.
What happens to thyroid function after the treatment?
In hyperthyroidism: thyroid activity gradually decreases over weeks to months. Most patients eventually become hypothyroid and need lifelong thyroid hormone replacement (levothyroxine). The exact transition is monitored with blood tests every 4 to 8 weeks for the first year.
In thyroid cancer: after total thyroidectomy plus RAI, all thyroid tissue has effectively been removed. You will need lifelong thyroid hormone replacement and ongoing follow-up with thyroglobulin blood tests, neck ultrasound, and sometimes whole-body I-131 scans to detect recurrence.
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.
Conclusion
Radioactive iodine therapy is one of the most established and effective nuclear-medicine treatments in use today — a targeted radiation delivered as a single capsule that specifically damages thyroid tissue while sparing the rest of the body. For hyperthyroidism, it is a one-time treatment that often produces a permanent result. For thyroid cancer, it is an essential part of comprehensive care after surgery. The radiation-safety precautions afterward are time-limited but real, and following them carefully protects your family while the treatment does its work.