The consequences of the tsunami that struck Japan on March 11, 2011 are truly devastating, with hundreds confirmed dead and many more still missing.  Unfortunately, the potential dangers that stem from this tsunami may stretch long into the future, as the Fukushima Nuclear Power Plant sustained damage from the tsunami as well.  Health authorities are now preparing to distribute iodine to individuals potentially exposed to radiation for the prevention of thyroid cancer, and I thought I’d take a moment to explain why.

Thyroid tissue is among the most radiation-sensitive tissues in the body.  The link between radiation exposure and thyroid cancer was better understood in the aftermath of the Chernobyl nuclear accident in 1986, where an increased risk of a common type of thyroid cancer called papillary thryoid cancer was found in people exposed to the disaster.  It appears that the Chernobyl radiation may have caused rearrangement of a cancer gene called the RET gene in the thyroid cells of these patients, leading to ongoing expression of this gene (this gene is normally not active in thyroid cells).

In the event of a nuclear accident, potassium iodide can be taken to decrease the thyroid’s uptake of radioactive iodine, thereby protecting the thyroid from developing these radiation-induced genetic alterations.  The ability of the iodine to protect the thyroid decreases within hours after the exposure, such that the iodine’s ability to protect decreases to 80, 40, and 7 percent at 2, 8, and 24 hours following exposure, respectively.   As potassium iodide prophylaxis may have a small risk of causing hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), particularly in people with enlarged thyroid glands, and because the risk of radiation-induced thyroid cancer is higher in younger people, the benefit is greatest in younger individuals.

In terms of medical radiation exposure, there is an increased risk of thyroid cancer in some patients who have received external beam radiation treatment (for example, for treatment of a cancer in the head or neck area), particularly if that exposure was at a younger age.  There is no evidence that radioactive iodine used for diagnosis or treatment of hyperthyroidism is associated with increased cancer risk.

Dr Sue Pedersen © 2011

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