Information for patients
This leaflet can be made available in other formats including large print, CD and Braille and in languages other than English, upon request.
Your doctor believes radioiodine (radioactive iodine) therapy is the best treatment for your overactive thyroid, also known as thyrotoxicosis. This leaflet includes generic guidelines and advice for patients receiving the treatment.
You will have a phone consultation with a member of the Medical Physics team before your appointment in order to tailor guidance that is personal to you.
This leaflet should answer most of the questions you may have about radioiodine at this stage. If you have more questions, please ask (see contact details below).
How does it work?
Most iodine in your diet goes to your thyroid gland. Radioiodine (which is normal iodine that has been made radioactive) will go to your thyroid in the same way.
Once in the thyroid, the radiation given off has the effect of ‘slowing down’ your thyroid.
What does treatment involve?
You will have to swallow a small (paracetamol sized) capsule with some water. You must stay in the department for half an hour after administration and then go home.
Is there any preparation before the treatment?
Your hospital doctor will give you instructions about your tablets. Some may need stopping before your treatment and may need to restart following your treatment. Please continue to take your present medication unless told otherwise.
It is important that you are not pregnant when you have your treatment, as radioiodine is harmful to unborn babies. At your appointment, we will ask about the possibility of you being pregnant, any contraception used and the date of your last period. We may also need to perform a pregnancy test if you are of childbearing age.
If you are currently breast-feeding, you will need to stop.
You must cut out foods that are rich in iodine for 1 week before treatment (and 2 days after administration).
What foods are rich in iodine?
Please cut down on these foods:
• Milk – a maximum of 5 teaspoons per day (which could be used in tea and coffee).
• Butter – limit to a very thin scraping (one teaspoon) per day.
• Cheese – 1 oz. or 25g once per week.
• Egg – one per week.
And completely cut out these foods:
- Seafood including fish, shellfish, seaweed, kelp, cod liver oil.
- Dairy produce other than above – such as yoghurt, ice-cream etc.
- Some dairy alternatives and plant milks may also be fortified with iodine and can contain sea salt – check the ingredients list if you are using alternatives.
- Food made with egg or yolk – such as custard, mayonnaise, egg pasta, egg fried rice etc.
- Milky coffee and tea, cappuccino, hot chocolate, malted milk drinks.
- Cakes and biscuits containing butter and eggs.
- Milk chocolate and white chocolate (dark chocolate is OK).
- Take-away meals and fast food/restaurant foods.
- Sea salt, iodised salt and pink Himalayan salt (normal table salt is fine).
- Vitamins and mineral supplements (unless advised to take by your medical team e.g. vitamin D), nutritional supplements and cough mixtures.
- Soya products such as soy sauce.
- Anything coloured red or pink with E127 (e.g. salami, spam, some sweets).
What happens after the treatment?
There are no side effects to the capsule and you can drive yourself home as normal. If someone else is driving you to your appointment, we ask that you sit in the backseat furthest away to reduce radiation dose to the driver.
It is ok to return home up to 1 hour after your treatment by public transport, avoid sitting next to pregnant women or small children.
You should arrange a thyroid function blood check 6-8 weeks after treatment and see your consultant 1 week after your blood check. The thyroid function check can be arranged via the haematology department at the hospital or via your GP.
You should start to feel some benefit within a few weeks of treatment. Your GP will continue to check your thyroid function for the rest of your lifetime, with advice from your consultant where necessary.
What if I feel very unwell?
It is extremely unlikely that the radioiodine treatment would cause any ill effects.
You should seek medical help as normal if you feel unwell and tell them you have had “radioiodine therapy for overactive thyroid”.
If you vomit within 48 hours of treatment, please contact the Medical Physics Department immediately for further advice and instruction as this will be radioactive.
Is the radiation harmful?
There is no evidence of harmful effects from the radiation. Radioiodine has been a successful treatment for overactive thyroid for over 80 years; the patients receiving this treatment are monitored carefully.
There is no recorded increased risk of developing cancer because of this therapy. The benefits of treatment vastly outweigh any risk.
How many treatments will I need?
Most people (9 out of 10) only need one treatment. A second or third treatment is occasionally required to receive the full benefit of treatment.
If required, further treatment can be administered 6 months after initial treatment. Your doctor will arrange blood tests to see if further treatment is necessary.
Will my thyroid become underactive?
5 out of 10 patients develop an underactive thyroid at some point in their life because of radioiodine treatment. This will be picked up via the regular blood tests you will have after treatment. If this happens, your doctor will give you thyroxine tablets
Will I be any danger to my family or friends?
The amount of radioiodine in your body will reduce with time. Some will also leave your body in bodily fluids such as sweat and urine.
People who are very close to you will receive a small radiation dose. You will still be able to perform your usual daily activities such as shopping, cooking and caring for pets.
It is important to keep the radiation dose to other people, especially young children and pregnant people, as low as possible.
One of the team will discuss this guidance with you on the phone and at the treatment appointment. Please raise any concerns or difficulties you may have with the restrictions and we will work through them with you. This may mean special arrangements that differ from those below.
Please share this information with close family members sharing your home so that they will be aware of these small risks and the precautions we have asked you to take.
If anyone has any concerns, please raise them – we will be happy to discuss concerns with anyone you choose to accompany you for your treatment.
To reduce risk you should:
- Stay at least 1 metre apart, and sleep apart from, pregnant women and children under the age of 5 for up to 23 days. Where this is not possible, you are allowed up to 15 minutes per day within 1 metre.
- Stay at least 1 metre apart, and sleep apart from, adults and older children for up to 13 days. Where this is not possible, you are allowed up to 15 minutes per day within 1 metre.
Your urine will be radioactive, with additional radioactivity in sweat, mucus, and saliva for the first 4 days following treatment.
In addition to the above, for the first 4 days, you should:
- Use the toilet sitting down and after using the toilet, shut the lid before flushing it twice.
- Use separate towels and face cloths to other people in the household.
- Keep your toothbrush separate from others in the household.
- Do not share cutlery and crockery unless it has been washed first.
- Avoid food preparation that involves a lot of handling of the food, e.g. making bread.
- Pay extra care to hand washing after using the toilet and before touching any food and drink.
Please change your bedsheets and towels at the end of the 4 days and then at the end of restrictions before you resume sharing with anyone else.
Clothing, sheets, towels etc. can be washed with the rest of the household’s items, similarly cutlery and crockery can be washed with the rest of the household’s items.
Will having this treatment stop me going to work?
If your work involves routinely sitting or standing within 1 metre of the same person or handling food items, you may need to stay off work for a number of days. A member of the Medical Physics team will discuss this with you prior to treatment.
If you work closely with children or pregnant people, use photosensitive material, or work with certain radiation equipment you may need to stop work for up to 2-3 weeks.
Are there any other potential restrictions?
- For people you see rarely (in shops, on public transport etc.) there are no restrictions.
- You must not donate blood for at least 6 months after treatment to maintain steady thyroid levels.
- Individuals of childbearing capacity must avoid becoming pregnant for 12 months following the treatment. If you do become pregnant within this time, you must seek advice from your GP immediately and inform them you have had radioiodine treatment. It is vital you have a blood test to check your thyroid hormone levels as this must be controlled during pregnancy.
Please be assured this treatment will not affect your fertility.
- You must stop breastfeeding before treatment and not restart.
- Individuals should avoid fathering children for 4 months following treatment.
You will receive exact details of your personal restrictions when you have your treatment. We also provide a yellow card to remind you about restrictions and how long they apply. Please carry the card on your person until the end of the restrictions. (Up to 23 days).
Travelling abroad or by aeroplane in the UK? Work in a port?
Some airports and ports have sensitive radiation detectors installed. The very small amount of radioiodine still in your body (even when all close contact restrictions have ended) may cause such a detector to alarm.
We advise you to keep your yellow restriction card and take it with you if you are by aeroplane or entering a port in the 6 months following your treatment. The Medical Physics Department may be contacted to confirm your treatment should you trigger any radiation alarm.
If you have any queries before or after the treatment telephone the Medical Physics Department and ask to speak to a nuclear medicine physicist.
Medical Physics Department, University Hospital of Hartlepool:
You can get more information about radioiodine treatment and thyroid disease from:
British Thyroid Foundation
One Sceptre House,
Hornbeam Square North,
Phone: 01423 810093
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This leaflet has been produced in partnership with patients and carers. All patient leaflets are regularly reviewed, and any suggestions you have as to how it may be improved are extremely valuable. Please write to the Clinical Governance team, North Tees and Hartlepool NHS Foundation Trust, University Hospital of North Tees, TS19 8PE or:
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Leaflet reference: PIL1430
Date for review: 28/06/2026