The thyroid gland, which is situated in the front of the neck, produces the thyroid hormones. These hormones contain iodine, so in order to make them the thyroid gland contains very high levels of iodine, which it concentrates from the blood.

There are five main types of thyroid cancer (based on how the cancer cells look under the microscope). These are papillary, follicular, anaplastic, medullary and lymphoma. Papillary and follicular tumours make up more than 80% of all thyroid cancers. Anaplastic tumours make up another 10% whilst medullary cancers and lymphomas of the thyroid are rare.

Because the cancer cells in papillary and follicular tumours are quite similar to normal thyroid cells they will often be able to take up iodine from the blood. Anaplastic, medullary and lymphoma cancer cells cannot do this.

Radio-iodine is a radioactive isotope that gives off a high level of radiation. The radiation from radio-iodine is called beta-radiation. Unlike some other forms of radiation beta-radiation only penetrates a very short distance. Radio-iodine also disappears from the body quite rapidly so that within a few days all traces of radiation will have disappeared.

This means that if a dose of radio-iodine is given most follicular and some papillary cancers will take up high concentrations of the radioactivity. This gives a high dose of damaging radiation to the cancer cells but, because of the short penetration of the radiation, there is little or no damage to normal tissue outside the thyroid gland.

Radio-iodine is therefore a useful treatment in the majority of follicular thyroid cancers and some papillary tumours. It is not usually given on its own but used with other treatments such as surgery or radiotherapy. The radiation destroys any normal tissue or thyroid cancer cells which may still be there after surgery, reducing the risk of the disease coming back and making follow-up and detection of recurrence easier if it does occur.

Radio-iodine is given as a capsule and rarely causes any immediate side-effects. The high dose of radiation to the thyroid gland does, however, damage the normal thyroid tissue and stops the production of the normal thyroid hormones. This means that patients who have radio-iodine treatment will have to take a tablet replacing these hormones for the rest of their life.