Underactive thyroid linked to pesticide exposure

NEW YORK (Reuters Health) - Exposure to certain types of pesticides could up the risk of thyroid disease in women, according to a new study of thousands of women married to licensed pesticide applicators.

Problems with the thyroid gland are more common among women than men, Dr. Whitney S. Goldner of the University of Nebraska Medical Center in Omaha and colleagues note in their report. The thyroid is located at the base of the throat and plays an important role in regulating the body’s energy use.

There is growing evidence for a link between exposure to pesticides and thyroid problems, the authors note. They studied more than 16,500 women living in Iowa and North Carolina who were married to men seeking certification to use restricted pesticides in those states during the 1990s.

Overall, 12.5 percent of the women reported having thyroid disease; 7 percent had underactive thyroid glands (hypothyroidism) and 2 percent had overactive thyroids (hyperthyroidism).

In the general population, Goldner and colleagues point out, the rate of diagnosed thyroid disease ranges from around 1 percent to 8 percent.

When they looked at 44 different pesticides, they found that women married to men who had ever used organochlorine insecticides -- such as aldrin, DDT, and lindane -- were 1.2 times as likely to have hypothyroidism. (Some of these pesticides are no longer used in the U.S. and elsewhere, although lindane is available in some states as a treatment for head lice.)

The risk of hypothyroidism for women exposed to fungus killers was 1.4-fold greater.

Specifically, they found that chlordane, an organochlorine pesticide, was associated with a 1.3-fold hypothyroid risk. The fungus killers benomyl and maneb/mancozeb were associated with tripled and doubled risk, respectively, and the herb killer paraquat nearly doubled the likelihood of hypothyroidism.

Maneb/mancozeb exposure increased women’s risk of hyperthyroidism more than two-fold; it was the only chemical studied that upped the risk of both hypothyroidism and hyperthyroidism.

It’s not clear why pesticides would be linked to thyroid problems. However, some studies have suggested that such chemicals have low levels of certain thyroid hormones.

Goldner’s team cautions that their study was not designed to tease out cause and effect. Similarly, because it was based on reports from subjects rather than more definitive information such as blood tests, further study is needed before calling the relationship definite.

SOURCE: American Journal of Epidemiology, online January 8, 2010.