The Radioactive Iodine Procedure
Radioactive iodine therapy uses a form of iodine the thyroid naturally absorbs to treat an overactive thyroid or thyroid cancer. It is usually given as a single capsule or drink.
This page explains what the procedure involves and what to expect afterwards.
Last medically reviewed: 15 November 2025
Before treatment
You may be asked to avoid iodine-rich foods and certain medications for a period beforehand so the thyroid takes up the treatment dose effectively. A pregnancy test is done where relevant, as radioactive iodine is not given during pregnancy or breastfeeding.
For some thyroid cancer patients, thyroid hormone may be paused or recombinant TSH used to stimulate any remaining thyroid tissue.
During treatment
The dose is swallowed as a capsule or a small drink — it is not painful and takes only moments. You may need to wait in the department briefly while radiation levels are checked.
The iodine travels through the bloodstream and concentrates in thyroid tissue, where it gradually destroys the overactive or cancerous cells while sparing the rest of the body.
After treatment
You will be given simple radiation-safety advice, such as keeping a short distance from pregnant women and young children for a few days and using separate towels. The exact precautions depend on the dose.
Drinking plenty of water helps clear the iodine and protects the salivary glands. Most people feel well and can resume normal activities quickly.
Follow-up
For an overactive thyroid, radioactive iodine often leads to an underactive thyroid, which is expected and managed with levothyroxine. Thyroid function is monitored regularly afterwards.
For thyroid cancer, follow-up combines blood tests and ultrasound on a schedule set by your specialist.
Frequently asked questions
Is radioactive iodine treatment painful?
No — it is swallowed as a capsule or drink. There is no needle and no discomfort during the treatment itself.
How long do radiation precautions last?
Precautions such as limiting close contact with pregnant women and young children usually last only a few days, depending on the dose. Your team will give specific instructions.
Will I need medication afterwards?
Most people become hypothyroid after radioactive iodine for an overactive thyroid and need levothyroxine. This is expected and easily managed.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
Ready to discuss your thyroid health?
Book a specialist consultation — in person or by teleconsultation.