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
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.