Did the Fukushima Nuclear Accident Increase Thyroid Cancer Rates?

Fukushima nuclear reactor radioactive iodine exposure thyroid cancer

In March of 2011, an accident at the Fukushima Daiichi Nuclear Power Plant in Fukushima, Japan—which was caused by a post-earthquake tsunami that hit the plant—triggered a release of radiation and exposure to radioactive materials in Japan and in areas downward of the nuclear plant.

Nuclear plant accidents such as the one at Fukushima and the 1986 Chernobyl accident result in the release of radioactive iodine-131.

Exposure to radioactive iodine-131 is a known risk factor for thyroid cancer, and the risks are greatest if the exposure occurs in infants, children, and adolescents. Is there cause for concern, and if so, what can be done?

The Research

A large spike in infant to adolescent thyroid cancer rates was seen starting around five years after the Chernobyl accident. The incidence was highest in areas such as Belarus, which was in the path of Chernobyl’s nuclear fallout, but whose population was unprotected by potassium iodide treatment. (Some areas downwind of Chernobyl, such as Poland, received preventive potassium iodide tablets, which protect the thyroid from absorbing radioactive iodine if taken in the hours before and after the exposure.)

Given the Chernobyl experience and widespread public concern in Japan, the Fukushima Health Management Survey was launched in July of 2011 to evaluate the risks of radiation exposure on the population.

The survey involved large-scale thyroid ultrasound screening of the population around Fukushima in an attempt to detect potential thyroid cancer.

The Japanese researchers were attempting to establish whether there is a proven relationship between the Fukushima reactor accident and any subsequent increases in the rates of thyroid cancer in the Fukushima population.

Early results were concerning to researchers, who found that half the subjects screened had thyroid nodules that could be currently cancerous or become cancerous in the future. One epidemiological assessment reported in 2015 that the rate of thyroid cancer in Fukushima's children was more than 600 per million, when the expected rate was 1 to 3 cases per million children.

According to researchers, however, while there is an increased incidence of thyroid cancer, it is much smaller than the significant increase that occurred after Chernobyl. This has led researchers to conclude that “exposure doses in Fukushima residents are much lower than those from the Chernobyl accident, and no strong evidence in support of the causal relation of thyroid cancer with radiation exposure in Fukushima is available so far.”

More Thyroid Cancer, or Better Detection in Fukushima?

Some Japanese researchers have pointed out that the advanced thyroid ultrasound being used for the Fukushima screening is capable of detecting the smallest thyroid nodules—known as microcarcinomas—and that previous estimates of the prevalence of thyroid nodules came from far less sensitive screening.

They argue that more nodules—and eventually, more thyroid cancer—will understandably be found in those exposed as children to Fukushima's fallout.

But they theorize that the increase in thyroid cancer rates is actually the result of the more sensitive and widespread screening taking place in Fukushima, versus an increase in the rate of thyroid cancer resulting from the nuclear accident. They are suggesting that more thyroid cancer will be found because researchers and Fukushima residents are looking for it, and using more sensitive screening tools to find it.

This issue mirrors similar debates taking place in the United States, where increased rates of thyroid cancer are being attributed to more sensitive detection tools capable of finding microcarcinomas, and not an actual increase in the incidence of thyroid cancer.

With regard to the Fukushima findings, Peter Kopp, MD, the editor of the journal Thyroid and Professor of Medicine, Division of Endocrinology, Metabolism, and Molecular Medicine, at Chicago’s Northwestern University, had this to say:

The careful study of the nuclear accidents in Chernobyl and Fukushima on health and societal issues continues to be highly informative. At this point, there is no clear evidence that the Fukushima accident has resulted in an increased incidence of thyroid carcinomas, a finding that contrasts with the observations after the Chernobyl accident. The relatively high incidence of thyroid malignancies detected through the screening of the Fukushima population highlights the challenges associated with screening programs.

However, any definite conclusion would be premature, and continuing observation of the Fukushima population, as well as detailed characterization of the genetic and pathological alterations in the detected thyroid carcinomas, remain important.

A Word From Verywell

While the Japanese researchers have not established any significant increase in thyroid cancer rates attributable directly to the Fukushima nuclear accident, they also indicate that more research is needed to explore the situation further.

In the end, further epidemiological studies will help to determine whether the exposure to radioactive iodine-131 after Fukushima was of a level sufficient enough to cause a demonstrable increase in thyroid cancer—such as occurred after Chernobyl—or if the increase is merely a byproduct of more rigorous, widespread, and sensitive thyroid cancer screening.


International Commission on Radiological Protection 2009 Application of the Commission’s Recommendations for the Protection of People in Emergency Exposure Situations. ICRP Publication 109. Ann. ICRP 39. 2009.

United Nations Scientific Committee on the Effects of Atomic Radiation. “Sources and effects of ionizing radiation.” UNSCEAR 2008 Report to the General Assembly with scientific annexes. Volume II, Scientific Annex D: Health effects due to radiation from the Chernobyl accident. United Nations, New York, NY. 2011.

Yamashita S, Thomas G (eds). Thyroid Cancer and Nuclear Accidents: Long-Term Aftereffects of Chernobyl and Fukushima. Academic Press, Elsevier, Inc., Cambridge, MA. 2017.

Yamashita, S et. al. “Lessons from Fukushima: Latest Findings of Thyroid Cancer After the Fukushima Nuclear Power Plant Accident.” Thyroid. Volume 28, Number 1, 2017 Mary Ann Liebert, Inc. DOI: 10.1089/thy.2017.0283