Radioactive iodine, explained

What radioactive iodine is

Radioactive iodine (RAI, or radioiodine, or I-131) is a treatment that uses a radioactive form of iodine to reduce or destroy overactive or cancerous thyroid tissue. It is given as a small capsule or drink - there is no injection and no surgery involved.

The thyroid has a unique ability: it absorbs iodine from the bloodstream to make thyroid hormone. RAI exploits this. When you swallow the radioactive iodine, the thyroid absorbs it just as it would ordinary iodine from food. The radiation then acts on the thyroid tissue from within, gradually reducing its ability to produce hormone over the following weeks and months.

Why people have it

RAI is used in two main contexts:

Hyperthyroidism - most commonly Graves' disease or toxic nodular goitre. When the thyroid is producing too much hormone and other treatments have not been the right fit, RAI is a definitive option that reduces thyroid activity. It is typically managed by a nuclear medicine or endocrinology team.

Thyroid cancer - after a total thyroidectomy (full removal of the thyroid), a higher dose of RAI is sometimes used to destroy any remaining thyroid cells. This makes it easier to monitor for any recurrence using thyroglobulin blood tests.

What to expect: the practical questions

How is it given? You swallow a capsule or liquid at a clinic visit - the appointment is usually brief. In the UK, this is typically done in a nuclear medicine department.

Is it safe? RAI has been used for over 70 years and has a long safety record for treating hyperthyroidism and thyroid cancer. Your team can discuss the evidence for your specific situation.

Will I feel ill afterwards? Most people notice little immediate effect. Some experience mild tenderness in the neck and temporary worsening of symptoms in the first few weeks as hormone stored in the thyroid is released. If symptoms feel severe, contact your team.

Do I need to go into isolation? Yes - for a period after treatment you will emit a low level of radiation, mainly through urine and saliva. You will need to take precautions to protect people around you, particularly young children and anyone who is pregnant. The length and specifics of isolation depend on your dose and your personal circumstances. Your nuclear medicine team will give you written, personalised guidance - follow their instructions, not general internet advice.

Low-iodine diet before treatment

Before a higher-dose RAI treatment (most commonly for thyroid cancer), some teams ask patients to follow a low-iodine diet for a period beforehand. The aim is to ensure the thyroid is "hungry" for iodine when the RAI dose is given, so it absorbs it as effectively as possible.

Whether this applies to you, for how long, and exactly what the diet involves are all questions your team will advise on for your specific situation. There is no single universal protocol. If you are told to follow a low-iodine diet, ask your team for written instructions, and see the iodine guide for broader context. Do not self-prescribe a low-iodine diet without your team's guidance.

Becoming hypothyroid afterwards - an expected outcome

For most people having RAI for hyperthyroidism, the treatment is very effective - sometimes more effective than intended. A common outcome is that the thyroid is reduced to a point where it can no longer produce enough hormone on its own, resulting in hypothyroidism.

This is not a complication or a failure - it is often an expected and entirely manageable outcome. Hypothyroidism after RAI is treated with levothyroxine, a daily tablet that replaces the hormone the thyroid no longer makes. Many people feel well on levothyroxine once levels are stable.

Your team will monitor your thyroid levels in the months after RAI and start levothyroxine if and when it is needed.

Radioactive iodine and Graves' eye disease

If you have Graves' disease, tell your specialist about any eye symptoms - grittiness, bulging, double vision, redness, or pain - before having RAI. In some people RAI can trigger or worsen thyroid eye disease, and that change is not always temporary - it can be new, or get progressively worse.

If your eye disease is active, your team may recommend a different treatment, or give RAI alongside a short course of steroids to protect your eyes. Two things genuinely lower the risk and are worth sorting beforehand: stopping smoking, and getting your thyroid levels under control. It is a conversation worth having openly with your team before you decide.

Pregnancy, fertility, and RAI

RAI is not suitable during pregnancy or breastfeeding, and you should avoid conceiving for a period after treatment. This applies to men too: the NHS advises women avoid getting pregnant for at least 6 months afterwards, and men avoid fathering a child for at least 4 months - but your team will confirm the exact timeframe for you. If you are planning a pregnancy, or think you might be pregnant, tell your team before any treatment is given.

Questions to ask your team before RAI

• How long will I need to be in isolation, and what does that mean practically for my household?
• Will I need a low-iodine diet before treatment, and if so what should I eat and avoid?
• What are the chances I will need levothyroxine afterwards, and when would that start?
• How will my thyroid levels be monitored after treatment?
• Are there any eye symptoms I should be aware of or flag?
• How long should I wait before trying for a pregnancy?

Bring these to your appointment - or use the questions tool to keep track of them.