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LAKE BUENA VISTA, Fla. -- Japanese survivors of the atomic-bomb attacks on Hiroshima and Nagasaki in 1945 who received low-dose radiation from the blasts do not appear to have any greater rates of thyroid disease than the general public, researchers reported here.
In reviewing outcomes on 2,668 subjects during the 2007-2011 period -- all of whom were younger than 10 at the time the bombs exploded over their cities -- Misa Imaizumi, MD, a researcher at the Radiation Effects Research Foundation in Nagasaki determined that 1.3% of the cohort had experienced thyrotoxicosis, 4.8% of the population had hypothyroidism, and 21.5% were positive for thyroid-related antibodies.
None of those percentages were significantly different than what would be seen in a non-exposed population, she said in her oral presentation at the International Thyroid Congress.
"Associations between atomic-bomb radiation exposure and thyroid dysfunction and autoimmune thyroid diseases were not found in survivors 60 years after their exposure in childhood," Imaizumi said.
The researchers were seeking individuals who were exposed to less than 5 Gy of radiation during the atomic bomb attacks in order to determine how low-dose radiation figured in long-term illness. The researchers specifically sought out individuals who were less than 10-years-old at the time because it is believed that radiation would affect children more than older persons. The mean age at exposure in this cohort was 4.1 years.
The mean radiation dose for the cohort was 0.182 Gy (range 0 to 4.04o). The mean age of the patients at the time they were examined for the study was 68 years. The cohort included 1,213 men and 1,455 women. Participants filled out questionnaires regarding previous thyroid diseases and were tested for various thyroid parameters. "We measured free T4, (thyroid-stimulating hormone) TSH, anti-thyroid peroxidase (TPO) antibody, and anti-Tg antibody," Imaizumi explained.
Thyrotoxicosis was found in 36 of the participants in the study, including 32 cases of hyperthyroidism or Graves' disease; 10 other participants had subclinical hyperthyroidism.
The researchers identified 129 cases of hypothyroidism, including 57 cases of antibody-positive hypothyroidism and 72 cases of antibody-negative hypothyroidism. Overall there were 386 cases of subclinical and overt hypothyroidism identified in the study.
Imaizumi and colleagues also reported on 573 cases in which the subjects were positive for antibodies -- 317 had antibodies for TPO, and 476 had antibodies to Tg.
"There have been several reports over the years that have looked at how low dose radiation exposure has impacted the thyroid on survivors, and the results have been similar to what Dr. Imaizumi found," said Mario Silva, MD, professor of endocrinology at the University of Milan. "What I would be interested in learning is whether the children of these survivors have been affected because sometimes these radiation-mediated problems will skip a generation. Hopefully, this group will follow the children and give us an answer in the future," he told MedPage Today.
"It is well known," Imaizumi said in her presentation, "that high-dose radiation exposure in the 10s of Gy to the thyroid induces hypothyroidism. However, the influences of radiation exposure by low to moderate -- less than 5 Gy -- on thyroid dysfunction and autoimmune thyroid disease have been debated."
Studies in Nagasaki during the 1980s and in Hiroshima and Nagasaki in the 2000s yielded conflicting results, and hence the research team sought to find a more definitive answer to the impact of this low-dose exposure to atomic bomb radiation.
Imaizumi said that various researchers have examined radiation exposure and their relationship to thyroid disease, but until her study radiation dose-response relationships for thyroid function had not been investigated.
The formula for determining the radiation dose included differentiation for the city where exposure took place, the sex of the child, and their age at the time of exposure. They also used a weighted measurement of radiation dose in Gy that accounted for the biological response to the sum of gamma thyroid dose and the neutron thyroid dose.
Imaizumi and Silva disclosed no relevant relationships with industry.
- Reviewed by F. Perry Wilson, MD, MSCE Assistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner