We sometimes receive questions about the possible risks to the thyroid from radiation released from nuclear reactors. This information has been produced to provide reliable guidance in the event of such a release. It is endorsed by British Thyroid Association.

How does radiation damage health and particularly the thyroid gland?

The thyroid needs iodine to produce thyroid hormones, which regulate the body’s metabolism. The thyroid contains an iodine pump mechanism to take up iodine from the blood. It cannot distinguish between regular (stable) iodine and radioactive iodine so it pumps in the iodine and then binds it. This means that radioactive iodine, when present, becomes concentrated and locked in to the thyroid gland.

Radiation can cause changes in the DNA structure, called mutations. Cancer results when mutations accumulate over time in the daughters of a cell where the DNA has been changed by exposure to radiation. (Cell daughters are formed when a parent cell divides into two or more daughter cells.)

Radiation released in a reactor accident may contain a radioactive form of iodine. When the thyroid gland absorbs too much radioactive iodine this can cause thyroid cancer. Taking normal iodine can reduce the risk to people because the normal iodine will be taken up by the thyroid gland preventing, to a large extent, the uptake of the radioactive iodine.

The largest recorded release of radioactive iodine was from the nuclear reactor accident at Chernobyl, Ukraine in 1986. This resulted in a marked increase in thyroid cancer in those who were young at the time (under 10 years old) in heavily contaminated areas of Ukraine, Belarus and Russia that are closest to the power plant.

What can people in affected areas do to protect their health?

Follow the local guidance. If requested to do so, stay inside with windows and doors shut. If offered potassium iodide, take it, but not before being told to do so or it will be less effective in blocking the uptake of radioactive iodine. If told not to eat locally-sourced food, don’t.

What is the purpose of taking potassium iodide tablets? Who should take them? When?

Potassium iodide (KI) is the same form of iodine found in, for example, iodised table salt. It floods the thyroid gland with iodine, and it therefore blocks the gland from taking up radioactive iodine. Stable iodine tablets should be taken when instructed to do so to be most effective.

Who should take potassium iodide?

The groups most at risk are young children (particularly children under 10) and pregnant women, but all those offered potassium iodide are advised to take it.

When should people take potassium iodide?

Tablets should be taken ideally before or within hours of release of any radioactive iodine and only when instructed by the proper authorities. People should normally take one dose a day during exposure to radioactive iodine and for one day afterwards. Anyone who has taken potassium iodide should see a doctor shortly afterwards.

The National Pharmacy Association (UK) has issued a statement saying that there is ‘no immediate necessity’ for UK pharmacists to dispense potassium iodide. It recommended that travellers returning home from Japan should seek the advice of their GP.

Taking in too much iodine can also be bad for your thyroid, so only take it when advised and under proper supervision.

Are there any other precautions that people in affected areas should know about?

Showering would remove any contamination on the skin, which would reduce the chance of contamination from putting fingers in mouths etc.

Crops grown in the vicinity of a nuclear reactor accident and milk from cows grazing locally can be a source of radioactive iodine. People in affected areas are therefore advised to follow information from government regarding consumption of locally produced milk or vegetables.

What type(s) of thyroid cancer may be caused by radioactive iodine being released as the result of a nuclear accident? Who is most at risk?

Papillary thyroid cancer is the most common form of thyroid cancer attributable to the Chernobyl accident (it is the most common even in the absence of radiation exposure). It is not yet known whether there will be a later increase in other types following the Chernobyl accident. Based on what has been learned from Chernobyl, those most at risk are children under 10.

Professor Gerry Thomas, Director of the Chernobyl Tissue Bank, emphasises that: ‘Only those aged under 10 and living in the contaminated areas of Belarus, Ukraine and Russia were at risk from radioiodine from Chernobyl – there’s no (scientific) evidence anywhere else. It is also worth mentioning that radioiodine does not stay around in the environment for very long – it has a half-life of approximately eight days. This meant that within three months of the Chernobyl disaster, radiation due to radioiodine had disappeared. Radiation is transient – it is different from a chemical poison where land can remain contaminated for years with little change in the amount of contamination.’

I am due to be treated with/have been treated with radioactive iodine for thyroid cancer. Should I be concerned?

Radioactive iodine has been used in the treatment of thyroid cancer for many decades. It is considered to be safe and is effective in killing off thyroid cells and therefore reducing the chance of a recurrence of thyroid cancer. People who have been treated for thyroid cancer need not be alarmed by what they read about the situation in Japan.

I am due to be treated with/have been treated with radioactive iodine for Graves’ disease. Should I be concerned?

People who have an over-active thyroid (e.g. Graves’ disease) and are being treated with a low dose of radioactive iodine should equally not be alarmed – radioactive iodine is given medically for an over-active thyroid to destroy sufficient cells to reduce the over-activity and does not carry with it an increased risk of thyroid cancer.

What is the incidence of thyroid cancer in the UK?

According to statistics from Cancer Research UK (CRUK), on average over 3,500 people were diagnosed with thyroid cancer in the UK during 2014-2016 and it is now the 17th most common cancer among women. 73% of thyroid cancer cases in the UK are in females, and 27% are in males.

What is the outlook for people with thyroid cancer?

Any diagnosis of cancer is alarming, but thyroid cancer is highly treatable, and most patients go on to live a full and normal life. However, thyroid cancer survivors do remain at some risk of recurrence and are recommended to have regular check-ups for life.

The prognosis for young patients who are diagnosed with thyroid cancer following an accidental release of radioactive iodine is no worse than it would be for young patients diagnosed with a non-radiation-induced form of thyroid cancer, i.e. it is very good.


Professor Gerry Thomas, Professor of Molecular Pathology, Imperial College, London and Director of the Chernobyl Tissue Bank

Professor Richard Wakeford from the Dalton Nuclear Institute, University of Manchester and visiting Professor in Epidemiology at the University of Manchester

Further information

Thyroid Cancer: For Patients, By Patients, Third Revised Edition 2017