A new University of California, Los Angeles study has found that in parts of California the rate of thyroid cancer patients with an advanced stage of the disease is well above the national average, prompting research into possible links to farming or radiation.

According to the study, 35 percent of Californians with thyroid cancer were not diagnosed until the disease had already spread to lymph nodes or other parts of the body, compared with 29 percent of people nationwide.

Dr. Avital Harari, a member of UCLA Jonsson Comprehensive Cancer Center and lead researcher on the study, said there was no geographic pattern to the California counties with the highest percentage of people with advanced thyroid cancer, prompting her to consider possible environmental factors.

"There's definitely something going on here, but we're not sure what explains it," Harari said. "To find the etiology of why its happening we need more research."

Harari said she was just beginning a second study that would examine potential links to farming, pesticides or radiation. Other studies have previously found some pesticides to be endocrine disruptors, she said, but none have established a link to cancer.

She said the counties with higher rates of advanced thyroid cancer could not be fully explained by socioeconomic factors or ethnicity, which are also known risk factors for advanced thyroid cancer.

The incidence of thyroid cancer has risen significantly over the past 30 years for reasons that are have not been fully explained.

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More than 60,000 people in the United States are diagnosed with thyroid cancer annually, most of them women between the ages of 40 and 60. The disease is treatable when detected early, but survival rates are much lower when it is found in advances stages.