Forty-five percent of hibakusha have developed some form of thyroid disease since their exposure to radiation from the atomic bombing of Hiroshima and Nagasaki in 1945, according to a study by the Radiation Effects Research Foundation released Wednesday.

The greater the level of radiation, and the younger the age of exposure, the greater the chance of developing a thyroid disease, the study, published in the Journal of the American Medical Association, says.

The researchers examined about 4,000 bomb survivors in the two cities between 2000 and 2003, and their study shows the effects of radiation are still serious 55 years after the attacks.

"It is necessary to continue monitoring the health of hibakusha carefully," said Misa Imaizumi, a researcher at the foundation.

The researchers studied 4,091 people who had a checkup at the foundation and agreed to participate in the thyroid testing.

The study found that 1,833, or about 45 percent, developed thyroid diseases in the past or were newly diagnosed with the diseases through ultrasound exams or cytoscopy.

Of the people with a thyroid condition, 2 percent had cancer, 5 percent had benign tumors and 8 percent had cysts filled with fluid. In addition, 28 percent have developed a chronic inflammation of the thyroid gland, or thyroiditis.

Of about 3,200 participants whose radiation exposure level is clearly known, the researchers found those exposed to a larger amount had a greater chance of developing thyroid diseases.