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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 explains how it works and what to expect.

Why does an overactive thyroid need treatment?

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

Is this treatment new?

No. It has been used since the 1940s and has now largely replaced surgery for treating overactive thyroids. In the USA it is often used as a first-line treatment, even before trying anti-thyroid medication (such as carbimazole).

How does the radioiodine work?

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.

If your doctor recommends radioiodine therapy, you will be made hypothyroid (underactive) for a short time, because you need high levels of TSH (thyroid stimulating hormone) for the therapy to work. This will happen either by not starting thyroid hormone pills after your thyroid gland is removed, or stopping your thyroid hormone pills if you are already taking them.

You will 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 ring the Nuclear Medicine Department on (03) 364‑0890.

Are there immediate side effects?

Not really. The iodine has no taste. Occasionally someone notices a mild sore throat or neck discomfort for up to a week after treatment.

Will my thyroid become underactive 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 will need regular blood tests, usually once a year. If your thyroid becomes underactive, you will need to take thyroid hormone (thyroxine) tablets every day. Thyroid hormone tablets are safe, well tolerated and better than taking anti-thyroid medication long term.

How much radioiodine will I get?

Your doctor will decide this, 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.

Does radioiodine produce cancer?

No. 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.

Does radioiodine make thyroid eye disease worse?

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. However, if you have eye disease then you might be given a course of treatment with a steroid like prednisone.

Can radioiodine lead to abnormal babies?

There is no risk to babies as long as we are careful.


No pregnant woman should have radioiodine. Premenopausal women should practise safe contraception for one month before treatment.

No breastfeeding woman should have radioiodine.

We also advise men to avoid fathering a child and women to avoid pregnancy for six months after treatment. Radioiodine treatment won't affect your future fertility.

Will it affect my children?

There is no evidence that children are affected by their parent's treatment. However we still recommend you take some precautions.

The doctor who prescribes the radioiodine will tell you how long after the radioiodine your thyroid will be radioactive. During this period (usually two to three weeks), you should not have children younger than 10 closer than two arms' length for more than a few minutes at a time. Don't stop essential contact such as cuddles, dressing or soothing but it is best to avoid kisses for 48 hours after the radioiodine.

Should I sleep on my own after treatment?

FDP businessmanIt is best, but not essential, that for several days after your treatment you sleep in a separate bed from your partner. Your doctor will discuss this with you.

Should I stay off work after treatment?

This really depends on your job. If you work with young 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 is all you need to do. The doctor will discuss your individual situation with you.

Can I take public transport home?

It would be better if you went home by car. If you need to take public transport, make sure you only sit with adults. Move to another seat if a child or pregnant woman sits down within two arms' length of you.

Does all the radioiodine go to the thyroid?

Your thyroid takes up most of the radioiodine. You remove the rest in your body fluids, particularly your urine and bowel motions (poos). During the first three days after your treatment, be sure to flush the toilet immediately, and then flush a second time, wipe up any spilled urine with a tissue and flush it away. Always wash your hands thoroughly afterwards.

If you have a cold, use tissues and ensure you dispose of them into a rubbish bin. Don't leave them lying around.

We also recommend that you don't cook food for others, and use your own crockery and cutlery for several days after the radioiodine treatment.

If you have any questions, please ask the doctor who sees you.

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Written by the Department of Endocrinology, Christchurch Hospital. Adapted by HealthInfo clinical advisers. Last reviewed March 2016.

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Page reference: 70799

Review key: HIRTT-70799