Women with greater blood levels of mercury are more likely to have higher amounts of proteins tied to autoimmune diseases such as arthritis.
Mercury is a naturally occurring metal released into the environment from rocks and soils, and in volcanic eruptions.
The chemical element is found in all organisms but most human exposure comes from eating fish, which contain the organic form known as methylmercury.
A new large study of more than 2,000 US women found an association between mercury exposure and elevated levels of a thyroid antibody that is often higher in patients with autoimmune diseases, conditions in which immune system mistakes some part of the body as a pathogen and attacks its own cells.
Women with the highest blood mercury levels were more than twice as likely to have elevated levels of thyroglobulin antibody compared to women with the lowest mercury levels, wrote Carolyn M. Gallagher and Jaymie R. Meliker in Environment International
Previous findings had tied mercury to certain health problems including heart and nervous immune diseases particularly in women of childbearing age and young children.
The new study has provided the first evidence suggesting that mercury affects the immune system through the thyroid. It, however, cannot prove that the element can cause thyroid or autoimmune disease.
Future studies may provide more data about potential links between long-term exposure to low levels of mercury and developing autoimmune disease, the authors added.