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HealthInfo Canterbury

Radioiodine treatment for thyrotoxicosis (overactive thyroid)

Doctors have recommended radioiodine (also called radioactive iodine) as a treatment for your overactive thyroid (thyrotoxicosis). This is done at Christchurch Hospital's Nuclear Medicine Department. This page has information about how it works and what to expect.

The reasons for treatment

If it isn't treated, an overactive thyroid can cause heart problems (particularly heart rhythm problems such as atrial fibrillation) and thin bones (osteopenia or osteoporosis).

How the radioiodine works

Radioiodine works by destroying overactive thyroid cells. You 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 need, depending on the size of your thyroid gland, your age, the severity of your thyroid disease, whether or not 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 overactive thyroids. In the USA it's often used as the first treatment, even before trying antithyroid medication (such as carbimazole).

If your doctor recommends radioiodine therapy, you'll be made hypothyroid (underactive) for a short time because you need high levels of TSH (thyroid stimulating hormone) for the therapy to work. To do this, you'll either not start thyroid hormone pills after your thyroid gland is removed or you'll stop your thyroid hormone pills if you're already taking them.

You'll also need to avoid iodine-rich foods, such as fish and seaweed and iodine-containing supplements, such as kelp for three weeks before treatment. The Nuclear Medicine Department 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 Nuclear Medicine Department on (03) 364‑0867.

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 patients. 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

At least a third of all patients will develop an underactive thyroid (hypothyroidism) in the year after treatment. After that, your thyroid may gradually become underactive, so you'll need regular blood tests, usually once a year. 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.

Radioiodine effect on babies

There's no risk to babies as long as treatment is done carefully.


No pregnant woman should have radioiodine. Premenopausal women should practise safe contraception for one month before treatment. Women routinely have a pregnancy test before they're treated.

No breastfeeding women should have radioiodine. Women should stop breastfeeding two months before radioiodine to make sure the radioactivity doesn't affect their breast tissue.

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

Radioiodine effect on children

There's 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 after the radioiodine your thyroid will be radioactive. 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 don't 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

FDP businessmanFor 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 down within two arms' length.

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 mucus from your nose, saliva and sweat. Use tissues to wipe your nose and make sure you put them in the rubbish – preferably not in your living areas. Don't 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 May 2020.

See also:


Overactive thyroid (hyperthyroidism)

Images courtesy of FreeDigitalPhotos.Net. Radioiodine by digitalart; businessman by imagerymajestic.

Page reference: 70799

Review key: HIRTT-70799