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Radioiodine for thyrotoxicosis (overactive thyroid)

Radioiodine mō te repe tenga hohe rawa


Pregnant women shouldn't have radioiodine. If you have not gone through menopause, you should practise safe contraception for one month before treatment. Women will routinely have a pregnancy test before they're treated.

Breastfeeding women shouldn't have radioiodine. You should stop breastfeeding two months before radioiodine to make sure the radioactivity doesn't affect your breast tissue.

Men should avoid fathering a child and women should avoid pregnancy for six months after treatment. Radioiodine treatment doesn't affect fertility.

Doctors have recommended that you have radioiodine (also called radioactive iodine) to treat your overactive thyroid (thyrotoxicosis). This is done at Christchurch Hospital Molecular Imaging and Therapy. This page has information about how radioiodine works, side effects and any precautions you need to take.

The reasons for your treatment

If it is not treated, an overactive thyroid can cause heart problems (particularly heart rhythm problems such as atrial fibrillation) and thin bones (osteopenia or osteoporosis). Although a medicine such as carbimazole can treat an overactive thyroid, often the overactivity will come back if you stop taking the medicine. Radioactive iodine usually cures an overactive thyroid permanently.

How the radioiodine works

Radioiodine works by destroying overactive thyroid cells. You'll have it in a drink of water or as a capsule (like an antibiotic capsule) as an outpatient. About 10% of people need more than one dose.

Your doctor will decide how much radioiodine you'll need. This will depend on the size of your thyroid gland, your age, how bad your thyroid disease is, whether you have thyroid eye disease and any other disease you have.

Radioiodine has been used since the 1940s and has now largely replaced surgery for treating an overactive thyroid.

You'll need to avoid iodine-rich foods such as fish and seaweed and iodine-containing supplements, such as kelp for three weeks before your treatment. The Department of Endocrinology will let you know what changes you need to make to what you eat when it sends your booking letter. If you have any questions about this, you can phone the Department of Endocrinology on (03) 364‑0890.

Side effects

The iodine has no taste. Occasionally, someone notices a mild sore throat or neck discomfort for up to a week after treatment.

Radioiodine doesn't cause cancer. It has been used for nearly 60 years and has treated more than a million people. Radioiodine is one of the safest treatments available for an overactive thyroid.

There is a very small risk that radioiodine may make thyroid eye disease worse if you have active eye disease, particularly if you smoke. If you have stable eye disease, the risk is low. But if you have eye disease, you might be given a course of treatment with a steroid like prednisone.

Underactive thyroid after treatment

Most people will develop an underactive thyroid (hypothyroidism) after treatment, although this may take months or years to develop. This is diagnosed through regular blood tests.

If your thyroid becomes underactive, you'll need to take thyroid hormone (thyroxine) tablets every day. Thyroid hormone tablets are safe, well tolerated and better than taking antithyroid medication long term. Some people's thyroid levels will go back to normal after the radioiodine. If this happens, you will not need to take any medication long term.

Radioiodine effect on children

There is no evidence that children are affected by their parent's treatment. But we still recommend that you reduce your children's radiation exposure to a minimum.

The doctor who prescribes the radioiodine will tell you how long your thyroid will be radioactive after your treatment. During this time, usually seven to 10 days, you shouldn't have children closer than two arms' length for more than a few minutes at a time. But you do not need to stop essential contact such as cuddles, dressing or soothing. It's best to avoid kisses for 48 hours after the radioiodine.

Precautions after treatment

For about one week after the treatment, it's best that you sleep in a separate bed from your partner. Your doctor will talk to you about this.

Whether you should stay off work after your treatment depends on your job. If you work with children or pregnant women, you should take some time off work. If you work with adults, keeping them at two arms' length from you for a few days may be all you need to do. The doctor will talk about this with you.

It's best that you go home by car after your treatment. If you have to take public transport, make sure that you only sit with adults. Move to another seat if a child or pregnant woman sits within two arms' length of you.

Your thyroid takes up much of the radioiodine and you remove the rest through your urine and bowel movements (poo). During the first three days after the radioiodine, flush the toilet immediately. Then flush the toilet a second time and wipe up any spilled urine with a tissue and flush it away. Always wash your hands well afterwards. It's best for men to use the toilet sitting down.

There will also be radioactivity in other body fluids, including saliva, sweat and mucus from your nose. Use tissues to wipe your nose and make sure you put them in the rubbish – preferably not in your living areas. Do not leave tissues lying around. Avoid cooking food for others and use your own crockery and cutlery for several days after your treatment.

If you have any more questions, ask your doctor.

Written by the Department of Endocrinology, Christchurch Hospital. Adapted by HealthInfo clinical advisers. Last reviewed July 2023.


See also:


Overactive thyroid (hyperthyroidism)

Page reference: 70799

Review key: HITHY-49064