N° 162
18 May 2005 

Iodine Deficiency and Supplementation Affect Thyroid Cancer Risk in Children Exposed to Radioactive Iodine

Radioactive iodine 131 linked to increased risk of thyroid cancer
Exposure to radioactive iodine 131 (I-131) in childhood is associated with an increased risk of thyroid cancer, and both iodine deficiency and supplementation appear to modify this risk, according to a new study in the May 18 issue of the Journal of the National Cancer Institute.

IARC led international study
To evaluate the risk of thyroid cancer after exposure to radioactive iodine in childhood and investigate factors that might modify this risk, Dr Elisabeth Cardis, of the WHO International Agency for Research on Cancer (IARC) in Lyon, France, and colleagues from Belarus, Russia, as well as from IARC, Germany, Italy, Japan, Lithuania, the UK and the USA conducted a case-control study of 276 thyroid cancer patients and 1,300 control subjects in Belarus and the Russian Federation who were younger than 15 at the time of the Chernobyl accident.

Huge increase in thyroid cancers in young people after Chernobyl disaster
The accident at the Chernobyl nuclear power plant in April 1986 resulted in widespread radioactive contamination, particularly in parts of Belarus, the Russian Federation, and the Ukraine. For people living in these areas, the main radiation dose was to the thyroid and came from exposure to I-131. (The thyroid uses iodine to make thyroid hormone.) It has been estimated that the thyroids of several thousand children in Belarus received radioactive I-131 doses of at least 2 Gy. (For comparison, doses to the thyroid from background radiation from natural sources are of the order of 1 to 2 mGy per year.) Said Cardis : "A very large increase in the incidence of thyroid cancer in young people was observed as early as 5 years after the accident in Belarus and slightly later in the Russian Federation and the Ukraine.
Although this increase has been attributed to fallout from the accident, questions remained about the magnitude of the risk related to I-131 and to the potential modifying effect of iodine deficiency, which was common in most of the affected areas at the time of the Chernobyl accident."

Role of radiation in the reported increase
Dr Cardis said the group "observed a strong dose-response relationship between radiation dose to the thyroid received during childhood and the risk of thyroid cancer. The risk was mainly attributable to I-131 and appeared to be similar in magnitude to that seen following external radiation exposures to X and gamma rays."

Role of iodine deficiency and supplementation in children exposed to I-131
This risk was three times higher in iodine-deficient areas than in other areas. Use of potassium iodide as a dietary supplement reduced that risk by a factor of three compared with no supplementation.

Both iodine deficiency and iodine supplementation appear to be important and independent modifiers of the risk of thyroid cancer after exposure to I-131 in childhood. Said Cardis: "This result has important public health implications in the case of exposure to radioactive iodines in childhood that may occur after radiation accidents or during medical diagnostic and therapeutic procedures." Indeed, stable iodine supplementation in iodine-deficient populations may reduce the subsequent risk of radiation-related thyroid cancer in these situations," the authors write.

Citation: Cardis E, Kesminiene A, Ivanov V, Malakhova I, Shibata Y, Khrouch V, et al. Risk of Thyroid Cancer After Exposure to 131I in Childhood. J Natl Cancer Inst 2005;97:TKTK.

Note: The Journal of the National Cancer Institute is published by Oxford University Press and is not affiliated with the National Cancer Institute. Attribution to the Journal of the National Cancer Institute is requested in all news coverage. Visit the Journal online at

Contacts :
IARC Communications, Dr Nicolas Gaudin
Dr Elisabeth Cardis

World Health Organization
International Agency for Research on Cancer

Organisation mondiale de la Santé
Centre international de Recherche sur le Cancer

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