The endocrine system is comprised of a number of glands in the body that do not have ducts and other similar structures. Glands in the endocrine system include the ovaries; the testes; the thyroid, parathyroid, adrenal, and pituitary glands; the pineal body; the pancreas; as well as cells of the gastrointestinal tract, and the placenta. What distinguishes endocrine glands from other glands is that endocrine glands secrete certain chemical called hormones and other glands produce other chemicals or fluids; for example, lacrimal glands secrete tears, salivary glands produce saliva, and sweat glands produce sweat.

Hormones (so-called "chemical messengers") produced by endocrine glands enter the capillaries (and lymph vessels) of the blood circulatory system. They travel through the bloodstream to specific "receptors" in target organs or systems where they can trigger their biological effects.

The endocrine system is important because it coordinates and regulates many essential body functions such as

• growth and maturation;
• behaviour;
• reproduction and embryo development;
• production, use and storage of energy;
• balance and maintenance of water and salt (electrolytes) in the body; and
• reaction to stimuli (e.g., fright, excitement).

• Some examples of hormones secreted by the endocrine system are:
• ACTH (adrenocorticotrophic hormone) - produced by the pituitary gland to influence the release of corticosteroid hormones from the adrenal glands,
• ADH (anti-diuretic hormone, vasopressin) - produced by the pituitary gland to stimulate water reabsorption by the kidney tubules
• adrenaline (epinephrine) and noradrenaline (norepinephrine) - two hormones formed in the adrenal gland that help the body to react (e.g., flight or fight, body defense mechanisms) under stressful conditions (they can increase the heart rate, blood pressure, blood sugar levels, blood clotting rate),
• estrogen - a female sex hormone produced by the ovaries,
• insulin - secreted by the pancreas and regulates the storage and use of carbohydrates in the body, and
• thyroid hormone - produced in the thyroid and influences the function of virtually every cell in the body (growth, development and metabolism).

The endocrine system controls many functions of the body, both immediate reactions and life-long functions. The hormones stabilize or balance functions in the normal body. In turn, the levels of hormones produced in the body are influenced by stimuli the body receives and are regulated by complex biological feedback systems. Any disruption to this balance can cause changes in the reproduction, development, growth, or behaviour that can affect that animal or human or their offspring or children.

Certain substances, both naturally-occuring and man-made, can affect the endocrine system. Some chemicals in plants (phytoestrogens) have estrogen-like effects. Certain drugs and environmental pollutants can either mimic or block actions of some hormones. When there is interference with the normal communication between the "messenger" hormone and the cell receptors, the chemical message is misinterpreted and an abnormal response is generated in the body.

Understanding the role the endocrine system (and the hormones that they produce) in the normal functioning of the body gives us some indication of the types of problems that might occur when proper endocrine function is disrupted. Moreover, many of these organs influence each other's activities, producing very complex interactions and making the effect of the disruptors exceedingly difficult to identify or predict.

Concerns have been raised over the possible roles of chemicals since many of the complex activities of the human body are controlled by the endocrine system. In addition, endocrine systems are present in most animals such as other mammals, non-mammalian vertebrates (e.g., birds, fish, amphibians, and reptiles) and invertebrates (e.g., insects, spiders, snails, crabs, lobsters, etc.).

Substances can disrupt the normal function of endocrine systems in three different ways:

1. They can mimic a natural hormone and lock onto a receptor with in the cell. The disruptor may give a signal stronger than the natural hormone, or a signal that occurs at the "wrong" time.
2. They can bind to a receptor within a cell and thus prevent the correct hormone from binding. The normal signal then fails to occur and the body fails to respond properly.
3. The disruptors can interfere or block the way natural hormones and receptors are made or controlled. This interference or blockage may occur only if relatively large doses of the substance are present.

If the endocrine disruptor stimulates or inhibits the endocrine system, then increased or decreased amounts of hormone may be produced. In some cases, even very small amounts of a disruptor may have a detectable effect.

The number of substances believed to act as endocrine disruptors is wide and varied, including both natural and synthetic materials. Concern arises because potential endocrine disruptors may be present in the environment at very low levels but still may be able to cause effects.

Many plants and animals produce substances that can have endocrine effects. Some of the substances are toxic but certain effects have proven beneficial in some circumstances. For example, some "endocrine disruptors" have been used to control fertility (birth control pills), to treat cancer (corticosteroids), and to treat psychiatric disorders and other medical conditions. Natural substances, such as sex hormones or phytoestrogens (plant chemicals having estrogen-like effects), can become concentrated in industrial, agricultural and municipal wastes. Exposure to these wastes may produce reactions in humans, wildlife, fish or birds.

Synthetic chemicals suspected as endocrine disruptors may reach humans and animals in a variety of ways. Some, such as pesticides, are released intentionally. Others are by-products of industrial processes and waste disposal - these include dioxins and PCBs - or are discharged from industrial or municipal treatment systems (See Table below)

Sources, category (type) and examples of substances that have been reported as potential endocrine disruptors include:

(Source: Environment Canada "Endocrine Disrupting Substances in the Environment", 1999)