Endocrine Disruptor - Endocrine System

Endocrine System

Endocrine systems are found in most varieties of animals. The endocrine system consists of glands that secrete hormones, and receptors that detect and react to the hormones.

Hormones travel throughout the body and act as chemical messengers. Hormones interface with cells that contain matching receptors in or on their surfaces. The hormone binds with the receptor, much like a key would fit into a lock. The endocrine system regulates adjustments through slower internal processes, using hormones as messengers. The endocrine system secretes hormones in response to environmental stimuli and to orchestrate developmental and reproductive changes. The adjustments brought on by the endocrine system are biochemical, changing the cell's internal and external chemistry to bring about a long term change in the body. These systems work together to maintain the proper functioning of the body through its entire life cycle. Sex steroids such as estrogens and androgens, as well as thyroid hormones, are subject to feedback regulation, which tends to limit the sensitivity of these glands.

Hormones work at very small doses (part per billion ranges). Endocrine disruption can thereby also occur from low-dose exposure to exogenous hormones or hormonally active chemicals that can interfere with receptors for other hormonally mediated processes. Furthermore, since endogenous hormones are already present in the body in biologically active concentrations, additional exposure to relatively small amounts of exogenous hormonally active substances can disrupt the proper functioning of the body's endocrine system. Thus, an endocrine disruptor can elicit adverse effects at much lower doses than a toxicity, acting through a different mechanism.

The timing of exposure is also critical. Most critical stages of development occur in utero, where the fertilized egg divides, rapidly developing every structure of a fully formed baby, including much of the wiring in the brain. Interfering with the hormonal communication in utero can have profound effects both structurally and toward brain development. Depending on the stage of reproductive development, interference with hormonal signaling can result in irreversible effects not seen in adults exposed to the same dose for the same length of time. Experiments with animals have identified critical developmental time points in utero and days after birth when exposure to chemicals that interfere with or mimic hormones have adverse effects that persist into adulthood. Disruption of thyroid function early in development may be the cause of abnormal sexual development in both males and females early motor development impairment, and learning disabilities.

There are studies of cell cultures, laboratory animals, wildlife, and accidentally exposed humans that show that environmental chemicals cause a wide range of reproductive, developmental, growth, and behavior effects, and so while "endocrine disruption in humans by pollutant chemicals remains largely undemonstrated, the underlying science is sound and the potential for such effects is real." While compounds that produce estrogenic, androgenic, antiandrogenic, and antithyroid actions have been studied, less is known about interactions with other hormones.

The interrelationship between exposures to chemicals and health effects are rather complex. It is hard to definitively link a particular chemical with a specific health effect, and exposed adults may not show any ill effects. But, fetuses and embryos, whose growth and development are highly controlled by the endocrine system, are more vulnerable to exposure and may suffer overt or subtle lifelong health and/or reproductive abnormalities. Prebirth exposure, in some cases, can lead to permanent alterations and adult diseases.

Some in the scientific community are concerned that exposure to endocrine disruptors in the womb or early in life may be associated with neurodevelopmental disorders including reduced IQ, ADHD, and autism. Certain cancers and uterine abnormalities in women are associated with exposure to DES in the womb due to DES used as a medical treatment.

In another case, phthalates in pregnant women’s urine was linked to subtle, but specific, genital changes in their male infants – a shorter, more female-like anogenital distance and associated incomplete descent of testes and a smaller scrotum and penis. The science behind this study has been questioned by phthalate industry consultants. As of June 2008, there are only five studies of anogenital distance in humans, and one researcher has stated "Whether AGD measures in humans relate to clinically important outcomes, however, remains to be determined, as does its utility as a measure of androgen action in epidemiologic studies."