Radioactive iodine treatment (Radioiodine (iodine-131) therapy for an overactive thyroid)
A one-off dose of radioactive iodine, taken as a drink or capsule, that is taken up by the thyroid and gradually calms down an overactive thyroid gland.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- It is a one-off dose of radioactive iodine that gradually calms an overactive thyroid; it is not instant and may take weeks to months to work.
- Most people eventually become underactive and need a daily thyroid hormone tablet for life, with regular blood tests.
- There are radiation-safety rules afterwards, including avoiding close contact with young children and pregnant people for a period set by your team.
- It must not be given in pregnancy or while breastfeeding, and both women and men are advised to delay conceiving for several months after treatment.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Can give long-term control of an overactive thyroid, often from a single dose
You are pregnant or breastfeeding, or there is any chance you could be pregnant.
Follow the contact and hygiene rules you were given. Drink plenty of fluids. You may notice mild neck tenderness.
Clear written radiation-safety instructions with exact timescales for each restriction.
Follow the contact and hygiene rules you were given. Drink plenty of fluids. You may notice mild neck tenderness.
Limited close contact with young children and pregnant people continues for the period your team set, which...
A clinic review and blood tests check how your thyroid is responding, and your medicines may be adjusted.
Thyroid levels can keep changing. Some people become underactive in this period and are started on a thyroid...
What is radioactive iodine treatment?
Radioactive iodine treatment (radioiodine, or I-131) is a treatment for an overactive thyroid (hyperthyroidism), most often caused by Graves' disease or by an overactive thyroid nodule or goitre. You swallow a small dose of iodine that carries a low level of radiation, usually as a single capsule or drink.
The thyroid is the only part of the body that soaks up iodine in this way. The radiation acts only over a very short distance, so it gradually reduces the number of overactive thyroid cells and calms the gland down, without a major effect on the rest of the body.
It is not a quick fix. It can take several weeks, sometimes a few months, for your thyroid levels to settle, and you may stay on tablets to control symptoms in the meantime. Over time, many people treated this way end up with an underactive thyroid, which is expected and is treated with a daily thyroid hormone tablet.
It is one of several options. Anti-thyroid medicines and surgery are the main alternatives, and the right choice depends on the cause, your eyes, your plans for pregnancy and your own preferences.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Treatment options for an overactive thyroid
| Option | What it involves | Things to weigh up |
|---|---|---|
| Anti-thyroid tablets | Carbimazole or propylthiouracil for many months | No radiation; can relapse when stopped; needs monitoring for side effects |
| Radioactive iodine | A single dose of radioiodine | Often a lasting fix, but commonly leads to an underactive thyroid and has radiation-safety rules |
| Thyroid surgery | An operation to remove part or all of the thyroid | Fast and reliable, but involves surgery, anaesthetic and scar; usually leads to thyroid hormone tablets for life |
There is no single best option for everyone. The right choice depends on the cause, your eyes, pregnancy plans and what matters to you.
Preparing for your treatment
- Talk through why radioiodine is being suggested, and what the alternatives (tablets or surgery) would mean for you.
- Tell the team if there is any chance you are pregnant; a pregnancy test is usually done first, and the treatment cannot go ahead if you are pregnant or breastfeeding.
- Ask whether to pause anti-thyroid tablets before treatment, as you may be asked to stop them for a few days either side.
- Discuss your eyes: if you have thyroid eye disease or are a smoker, your team may advise extra steps or a different option, as radioiodine can sometimes worsen eye disease.
- Plan the days afterwards: who you live with, whether you care for young children, and any travel, so you can follow the radiation-safety rules.
- Ask about stopping smoking, which is strongly advised, especially if your eyes are affected.
- Bring a list of your medicines and any recent thyroid blood tests and scans.
What happens
You attend a nuclear medicine or endocrine department. After checking it is safe to go ahead, you swallow the radioiodine as a capsule or drink. The appointment itself is short.
Most people with an overactive thyroid go home the same day with written radiation-safety instructions. Higher doses, used more often for thyroid cancer, may need a short stay in a special shielded room until the radiation level falls.
You will be given clear rules about contact with other people, especially young children and anyone who is or might be pregnant, and about hygiene at home. The team will tell you exactly how long each rule applies, because this depends on your dose.
Over the following weeks your thyroid levels are rechecked, and your medicines are adjusted as the treatment takes effect.
Is this treatment right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- You are pregnant or breastfeeding, or there is any chance you could be pregnant.
- You have active, moderate-to-severe thyroid eye disease, where another treatment may be safer for your eyes.
- You cannot follow the radiation-safety rules, for example you are the sole carer of very young children with no alternative arrangements.
- Your overactivity is better treated another way, such as urgent surgery for a very large goitre causing pressure symptoms.
Delay or rearrange if…
- There is any possibility of pregnancy until this has been excluded.
- Your eyes are actively inflamed and your team wants to protect or treat them first.
- You are very unwell with severe overactivity that needs to be stabilised first.
- You cannot yet arrange childcare or living arrangements to follow the safety rules.
Alternatives to discuss
- Anti-thyroid medicines such as carbimazole or propylthiouracil, sometimes long term.
- Thyroid surgery to remove part or all of the gland.
- Watchful management with medicines while you decide, in selected cases.
- A different treatment if you have significant thyroid eye disease or are planning pregnancy soon.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Can give long-term control of an overactive thyroid, often from a single dose
- Avoids an operation and a general anaesthetic
- Does not involve a surgical scar
- Can be repeated if the first dose does not fully control the overactivity
- Has been used for many decades, so its effects and safety are well understood
Risks & complications
- Becoming underactive (hypothyroid) over time, needing a daily thyroid hormone tablet for life
- A slow response, so symptoms and tablets may continue for weeks to months
- Temporary neck tenderness or mild swelling of the thyroid
- Needing radiation-safety restrictions that disrupt normal family and work life for a period
- Not enough effect from one dose, so a second dose or another treatment is needed
- A short-term flare of overactivity soon after treatment
- New or worsening thyroid eye disease, particularly in smokers
- Temporary changes in taste or dry mouth
- A significant worsening of thyroid eye disease affecting vision, which needs urgent specialist care
- Effects on fertility are not expected at the doses used for an overactive thyroid, but conceiving too soon is advised against
The most important issues are that an underactive thyroid is a likely outcome rather than a complication, that it must be avoided in pregnancy and breastfeeding, and that it can worsen thyroid eye disease, especially in smokers. Ask your specialist how these apply to you, what your follow-up plan is, and exactly how long the radiation-safety rules last in your case.
Published figures to discuss
Outcomes vary with the cause, the dose given, gland size and individual response, so exact numbers are hard to pin down for any one person. The chance of ending up underactive rises over time, and many centres deliberately aim for this to give reliable control. Worsening of thyroid eye disease is more likely in smokers. Because robust single figures are not meaningful here, we describe the pattern rather than invent precise percentages.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Hypothyroidism after treatment | Common over time and sometimes the intended endpoint | Patients need thyroid blood-test follow-up and should expect that lifelong levothyroxine may be needed. | Guide sourcesClinical context |
| Worsening thyroid eye disease | Recognised, especially in smokers and active eye disease | Eye symptoms, smoking status and steroid prophylaxis decisions should be discussed before treatment. | Guide sourcesClinical context |
| Pregnancy and breastfeeding | Contraindicated during pregnancy and breastfeeding; pregnancy should be delayed afterwards | Local nuclear medicine instructions usually specify the waiting period and radiation precautions. | Guide sourcesClinical context |
| Radiation precautions after treatment | Required for days to weeks depending on dose and circumstances | Patients need written instructions about close contact, children, pregnancy exposure, travel and work. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery in the usual sense; the main thing afterwards is following the radiation-safety rules and waiting for your thyroid levels to settle while you are monitored.
- Mild neck or throat tenderness for a few days
- Symptoms of an overactive thyroid easing only slowly over weeks
- Tiredness as your thyroid levels change
- Needing to keep your distance from young children and pregnant people for a set period
- Dry mouth or a temporary change in taste
Aftercare
- Follow your written radiation-safety instructions on contact with others, sleeping arrangements, public transport and hygiene for exactly as long as advised.
- Avoid close, prolonged contact with young children and anyone who is or might be pregnant for the period your team specifies.
- Drink plenty of fluids and flush the toilet carefully in the first days, as instructed.
- Take any anti-thyroid or other medicines exactly as your team advises, including when to restart them.
- Attend your follow-up blood tests, even when you feel well, so an underactive thyroid is picked up early.
- Keep your treatment record, as you may set off radiation detectors at airports or ports for a few months.
- Report eye symptoms such as bulging, double vision or pain promptly, especially if you have thyroid eye disease.
- Written radiation-safety instructions understood and kept
- A plan for childcare if you look after young children
- Sleeping and contact arrangements at home sorted for the restriction period
- Follow-up blood test appointment booked
- Treatment record card kept for travel
- Clear contact number for the department saved
- Pregnancy-avoidance timescale agreed and understood
⚠ Get urgent help if…
- New or worsening bulging of the eyes, double vision, eye pain or any change in vision — seek specialist eye advice urgently
- A racing, pounding or irregular heartbeat, or feeling very unwell soon after treatment
- Severe neck swelling or difficulty breathing or swallowing
- A high temperature, sore throat or feeling generally unwell if you are taking carbimazole or propylthiouracil (these can rarely affect blood cells)
- Symptoms of an underactive thyroid such as marked tiredness, weight gain, feeling cold or low mood, which need a blood test
- Any concern that you might be pregnant after treatment
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
A good result is a thyroid that is no longer overactive. This can take weeks to months, and you may need ongoing monitoring and medicine adjustments while it settles. For many people the overactivity is controlled long term from a single dose.
Becoming underactive is common and is not a failure of treatment; it is treated with a daily thyroid hormone tablet and regular blood tests. Radioiodine cannot guarantee that you will end up with a perfectly normal thyroid without any tablets, and it does not treat thyroid eye disease.
Radioiodine often gives lasting control of an overactive thyroid, but your thyroid function can keep changing for years, which is why lifelong blood tests are advised. If the gland was not fully treated, the overactivity can persist or come back and a further dose or another treatment may be considered.
Related tests, treatments or support
Anti-thyroid tablets are often used before and sometimes after radioiodine to control symptoms while it takes effect. If you have thyroid eye disease, your team may consider steps to protect your eyes around the time of treatment. Radioiodine for an overactive thyroid is different from the higher-dose radioiodine used after surgery for thyroid cancer.
Follow-up & long-term care
You should have a clinic review and blood tests within about six weeks, then ongoing checks as your thyroid settles. Because an underactive thyroid can appear months or years later, lifelong monitoring of your thyroid levels is recommended, usually through your GP or endocrine team.
- Lifelong thyroid blood tests, even when you feel well
- A daily thyroid hormone tablet for life if you become underactive
- Prompt review of any eye symptoms, especially if you smoke or have thyroid eye disease
- Discussing pregnancy plans with your team before trying to conceive
Repeat, follow-on and what comes next
- A second dose, or a switch to surgery, may be needed if one dose does not fully control the overactivity.
- Becoming underactive is common and is managed with a daily thyroid hormone tablet rather than being seen as a complication.
- Thyroid levels can keep changing for years, so the treatment plan and tablet dose may be adjusted over time.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- Clear written radiation-safety instructions with exact timescales for each restriction.
- A booked follow-up blood test and clinic review, with a named contact for problems.
- A plan for lifelong thyroid monitoring and prompt treatment if you become underactive.
- Specific advice on pregnancy avoidance and on reporting eye symptoms urgently.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- The specialist (endocrinologist and nuclear medicine) fees
- Whether scans are needed to plan or calculate the dose
- Whether the dose can be given as an outpatient or needs an overnight stay in a shielded room
- Blood tests and follow-up appointments while your thyroid settles
- Anti-thyroid medicines used before and after treatment
- Any extra care needed if you have thyroid eye disease
- The specialist and facility fees for giving the treatment
- Any planning scans or dose calculation
- Follow-up blood tests and clinic reviews, and how many are included
- What happens, and what it costs, if a second dose is needed
- Medicines needed before and after treatment
- Who to contact for problems and how eye disease would be managed
On the NHS? Radioiodine for an overactive thyroid is commonly available on the NHS when clinically indicated; private care is mainly used for speed, choice of specialist or self-pay.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Not being told that an underactive thyroid is a likely outcome needing lifelong tablets and blood tests.
- Vague or missing radiation-safety instructions, especially about young children and pregnancy.
- No discussion of the effect on thyroid eye disease, particularly for smokers.
- No clear advice on how long to avoid conceiving or fathering a child.
- Treating radioiodine as instant when it can take weeks to months to work.
Marketing red flags
- Describing it as a quick or one-and-done cure with no mention of becoming underactive.
- Playing down the radiation-safety rules or the need for lifelong monitoring.
- Not mentioning pregnancy and breastfeeding restrictions clearly.
- Ignoring the link with thyroid eye disease and smoking.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- Why is radioiodine being recommended for me rather than tablets or surgery?
- How likely am I to become underactive, and what would that mean day to day?
- Exactly how long will each radiation-safety rule apply in my case?
- How long should I avoid trying for a baby or fathering a child?
- Could this affect my eyes, and what will you do to protect them?
- What is my follow-up and blood-test plan, and who do I contact with problems?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my treatment, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this treatment not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Will radioactive iodine make me radioactive to other people?
Can I have it if I am pregnant or breastfeeding?
Will I definitely become underactive?
Is it available on the NHS?
Does it cause cancer?
What if I have thyroid eye disease?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: NHS — Overactive thyroid (hyperthyroidism): treatment British Thyroid Foundation — Radioactive iodine (RAI) therapy resources Cambridge University Hospitals NHS — Radioiodine for hyperthyroidism Society for Endocrinology — patient and clinical information Cure rates after a single dose of radioactive iodine — PMC review
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
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