Radioactive Iodine

Radioactive iodine (RAI) is given by some endocrinologists who have additional training in the procedure. RAI therapy is a tablet taken on one occasion to treat hyperthyroidism (overactive thyroid) and certain types of thyroid cancer. Therapy is brief and is just a matter of swallowing a tablet. The preparation for the procedure and the precautions you need to take after RAI are more time consuming than the treatment itself.

The thyroid gland needs iodine to produce thyroid hormones. Thyroid cells and some thyroid cancer cells contain a symport that allows iodine to be taken up by the cell. While most thyroid cancer cells do not produce thyroid hormone many thyroid cancer cells still contain the symport.  When radioactive iodine (I-131) is digested, it travels through the circulatory system and is actively taken up by the thyroid. RAI works by targeting and destroying thyroid cells, including some thyroid cancer cells.  RAI emits radiation and destroys the thyroid cells. 

There are three main causes for hyperthyroidism:

1. Graves’ disease
2. Toxic multinodular goitre
3. Autonomous hot nodule

Treatments for hyperthyroidism include medications, RAI, surgery and radiofrequency ablation.

RAI is sometimes recommended when:

1. Hyperthyroidism returns after taking carbimazole.
2. You are unable to take carbimazole or propylthiouracil (PTU).
3. You are not healthy enough for surgery.

RAI usually leads to permanent destruction of your thyroid, curing your hyperthyroidism. After RAI you are very likely to need to take thyroid hormone tablets (thyroxine) every day for the rest of your life.

RAI is not recommended if you have:

1. Graves’ disease with marked eye problems (significant ophthalmopathy)
2. Very high thyroid hormone levels.
3. You are pregnant or breast feeding.

Low dose RAI is usually used to treat hyperthyroidism. The dose given is up to about 30 millicuries (mCi) which is equivalent to 1100 megabecquerels (MBq). RAI cures hyperthyroidism about 80% of the time. RAI takes about 2-6 months to cure hyperthyroidism.  

RAI is taken up by normal thyroid cells and some thyroid cancer cells. Many papillary and follicular thyroid cancers take up RAI. Papillary thyroid cancers that take up iodine are called iodine avid. Medullary and anaplastic thyroid cancers do not take up RAI, therefore RAI does not work for these patients.  

The American Thyroid Association (ATA) divides patients with papillary and follicular thyroid cancer into three risk groups-high, intermediate and low risk.

Most patients in the high-risk group and some patients in the intermediate risk group may benefit from RAI. The decision to use RAI is made in a multidisciplinary meeting (MDM) on a case by case (individual) basis. About 20% of patients with papillary and follicular thyroid cancer receive RAI. If RAI is recommended it is usually given about 6-12 weeks after surgery. RAI is usually deferred for 8 weeks following a CT scan, because the iodine in the contrast agent given at the time of the CT scan interferes with the effectiveness of the RAI. It takes time for this iodine to be excreted from the body.  High dose RAI is usually used to treat thyroid cancer. The dose given is usually in the range 100-200 mCi (3700-7400 MBq). Higher doses are used for the higher risk group.

A blood test is performed a day or two prior to RAI therapy. This blood test is done to measure the levels of TSH, fT4, thyroglobulin (stimulated thyroglobulin level) and thyroglobulin antibodies. An immediate pretreatment low dose RAI scan called a thyroid cancer uptake study may be performed. This helps identify the site and extent of any remaining thyroid tissue and thyroid cancer. The percentage uptake of RAI is also determined. Information from the thyroid cancer uptake study and the stimulated thyroglobulin level may be used to determine the dose of RAI to be given.

A second scan is done 48-72 hours following the high dose RAI therapy. This scan is called a whole-body scan and is thought to be more accurate in determining the location and percent uptake of the radioactive iodine. 

Your endocrinologist (hormone doctor) will give you detailed instructions on what you need to do prior to getting RAI. Please follow these details to ensure the RAI is most effective.

Instructions may include:

1. Stopping thyroxine for 3-4 weeks prior to RAI
2. Stopping carbimazole at least 1 week prior to RAI
3. Following a low-iodine diet for 3-4 weeks prior to RAI
4. Receiving injections of Thyrogen (recombinant thyroid stimulating hormone) to increase the amount of radioactive iodine your thyroid will absorb.
5. If you’re currently breastfeeding and are willing to stop, you’ll need to stop at least six weeks before the procedure.
6. Having a pregnancy test just prior to RAI.
7. Having a thyroid cancer uptake scan the day before RAI to check for any thyroid tissue or thyroid cancer.
8. Having a blood test to check the levels of TSH, fT4, thyroglobulin (stimulated thyroglobulin level) and thyroglobulin antibodies.

If you are having RAI for thyroid cancer, your endocrinologist will ask you to stop thyroxine 3-4 weeks prior to RAI. Alternatively, you can have two injections of Thyrogen (recombinant thyroid stimulating hormone). Thyrogen is very expensive and at the time of writing (2025), Thyrogen is not funded in New Zealand.

When you stop thyroxine, your TSH (thyroid stimulating hormone) level rises, and you slowly become hypothyroid. By 3-4 weeks you are very hypothyroid, your TSH level is very high, and your thyroid cells are very hungry for iodine. This is the ideal time for RAI to be given. The thyroid cells take up RAI and subsequently die.

For the first week or so after stopping thyroxine you feel OK, but after 3-4 weeks you feel very tired, lacking in energy and lethargic. After RAI your endocrinologist will tell you when you can restart thyroxine (usually about 2-3 days after RAI). It then takes a few weeks to start feeling normal again.

The way you feel being hypothyroid is one of the toughest parts of RAI.

RAI is contraindicated in pregnancy and to be avoided. It is also recommended that women do not get pregnant for 6-12 months after RAI. 

After RAI your body fluids (urine, sweat, saliva, faeces) emit radiation. Your endocrinologist will advise you what precautions you should take after RAI. The precautions depend on the dose of radiotherapy and your personal situation.

Precautions may include:

1. staying away from people for anywhere from 3-14 days.
2. staying away from pregnant women and children.
3. sleeping alone.
4. not breast feeding.
5. drinking plenty of fluids-this helps flush out the radiation.
6. avoid preparing food for others.

The above precautions are to minimise radiation exposure to others. The precautions are centred around distance, time and hygiene.  

1. Sore throat
2. Swelling of the salivary glands (in front of the ear and below the jaw)
3. Dry mouth
4. Dry eyes
5. Gastritis
6. Taste changes.

1. Low sperm count and male infertility for up to two years after treatment
2. Irregular periods for up to one year after treatment
3. Extremely high thyroid hormone levels (thyroid storm)
4. Slightly increased risk of developing leukaemia, salivary gland cancer, stomach cancer, thyroid cancer

Incidence of these risks or complications are low. 

Summary of RAI

Radioactive is an effective treatment for hyperthyroidism and some patients with papillary and follicular thyroid cancer. It is given by some endocrinologists.