What are Endocrine Disruptors?

Endocrine disruptors are chemicals which interfere with endocrine system function. An endocrine system is found in nearly all animals, including mammals, non-mammalian vertebrates (like fish, amphibians, reptiles, and birds), and invertebrates (like snails, lobsters, insects, and other species). The endocrine system consists of glands and the hormones they produce that guide the development, growth, reproduction, and behavior of human beings and animals. Some of the endocrine glands are the pituitary, thyroid, and adrenal glands, the female ovaries and male testes. Hormones are biochemicals, produced by endocrine glands, that travel through the bloodstream and cause responses in other parts of the body. The Endocrine Disruptor Screening Program will focus on the estrogen, androgen and thyroid hormones.

Estrogens are a group of chemically similar hormones responsible for female sexual development; estrogen is produced mainly by the ovaries, but also by the adrenal glands. Androgens are substances, usually hormones, responsible for male sex characteristics. Testosterone, the sex hormone produced by the testicles, is an androgen. The thyroid gland secretes two main hormones, thyroxine and triiodothyronine, into the bloodstream. These hormones stimulate all the cells in the body.

Hormones can produce both positive and negative effects. For example, some types of breast cancer are exacerbated by estrogen, but studies also indicate that estrogen has a protective effect in combating heart disease and osteoporosis-related fractures in older women.

Click here for a more detailed description of the endocrine glands and their functions.

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Disruption of this complex system can occur in various ways. For example, some chemicals may mimic a natural hormone,"fooling" the body into over-responding to the stimulus or responding at inappropriate times. Other chemicals may block the effects of a hormone in parts of the body normally sensitive to it. Still others may directly stimulate or inhibit the endocrine system, causing overproduction or underproduction of hormones. Certain drugs are used to intentionally cause some of these effects, such as birth control pills.

An example of the devastating consequences of exposure of developing animals, including humans, to endocrine disruptors is the case of the potent drug diethylstilbestrol (DES), a synthetic estrogen. Medical doctors prescribed DES to as many as five million pregnant women to block spontaneous abortion prior to DES being banned in the early 1970's. DES was prescribed in the mistaken belief that it would prevent miscarriage and promote fetal growth. It was discovered after the children went through puberty that DES affected the development of the reproductive system and caused vaginal cancer. Since then, Congress has improved how drugs and other chemicals are evaluated and regulated – requiring that an endocrine disruptor screening program be established is a recent and significant step.

Although regulatory policy concerning endocrine disruptors is still evolving, EPA has already taken regulatory action on some chemicals of concern through the pesticide and toxic substances programs. Organochlorine compounds, such as polychlorinated biphenyls (PCB’s) and chlorinated pesticides, have long been problematic in the environment for a number of reasons, and many of them (like DDT) have endocrine activity.

The term organochlorine refers to chemical compounds that have a chlorinated hydrocarbon structure, that is, they are formed from atoms of hydrogen, carbon, and chlorine. Although their effect may be much weaker than the body’s natural hormones (like estrogens, androgens, and thyroid hormones), they are nonetheless suspected of disrupting the endocrine system, resulting in harmful effects like reproductive and developmental defects and certain cancers. EPA has banned PCB’s, dieldrin, DDT, chlordane, aldrin, kepone, mirex, endrin, and toxaphene. Organochlorine pesticides still registered for use in the United States include endosulfan, lindane, methoxychlor, dicofol, dienochlor, and heptachlor. However, their use is very restricted and most are scheduled for priority pesticide re-registration review. They will likely be among the first compounds to be screened in the Endocrine Disruptor Screening Program.