Study finds link between pesticides and thyroid cancer risk in Central California

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New research has found that 10 out of 29 reviewed pesticides were associated with thyroid cancer, including several of the most widely used ones in the United States.

The study, published in The Journal of Clinical Endocrinology & Metabolism was conducted by investigators at the University of California, Los Angeles Health Sciences. Through this research they examined the association between exposure to pesticides, including 19 that were found to cause DNA cell damage, and the risk of thyroid cancer. The researchers hypothesized that pesticide exposure may be a missing link requiring further investigation.  

The authors performed a case-controlled study using thyroid cancer cases from the California Cancer Registry between 1999 and 2012 and controls sampled in a population-based manner. According to the study, participants who were evaluated were diagnosed with thyroid cancer, lived in the study area when diagnosed, and were aged 35 or older. Control subjects of the same age were recruited from the same geographic area and were eligible if they had been living in California for at least five years before the research interview. The study sample included 2,067 thyroid cancer cases and 1,003 control participants. 

The researchers examined residential exposure to 29 agricultural-use pesticides known to cause DNA damage or endocrine disruption. They used a validated geographic information-based system to generate exposure estimates for each study participant.

The study found that within a 20-year period, 10 out of 29 reviewed pesticides were associated with thyroid cancer, including several of the most widely used ones in the U.S. These included paraquat dichloride, glyphosate, and oxyfluorfen.

Additionally, the risk of thyroid cancer increased proportionally to the total number of pesticides subjects were exposed to 20 years before diagnosis or the research interview. In all models, paraquat dichloride was associated with thyroid cancer.

“The incidence of thyroid cancer has been increasing exponentially over the course of the last few decades,” said Avital Harari, MD, a member of the UCLA Endocrine Surgery team, and the study’s corresponding author and principal investigator in a statement. “Additionally, the risk of advanced thyroid cancers, which can increase risk of mortality and cancer recurrence, has been found to be higher in the state of California as compared to other states. Therefore, it is essential to elucidate risk factors for getting thyroid cancer and understand potentially alterable causes of this disease in order to decrease risks for future generations."