Hormones are the body's chemical messengers. Released in minute quantities, they regulate normal body function and development. In mammals and birds, hormone synthesizing glands include the ovaries, testes, pancreas, adrenals, thyroid, parathyroid and thymus glands. Together these glands comprise the endocrine system. Growth, embryonic development, and reproduction are examples of critical biological functions that are controlled by the endocrine system.
Some chemical pollutants are structurally similar to hormones and thus can mimic hormone signals.
These chemicals can cause the over or under production of hormones or blockage of the action of a hormone which can affect the normal development and function of the animal. Development of an embryo is regulated through precisely timed hormone signals that trigger gene expression. The wrong hormone signal will result in a change in the genetic expression and an error in the development and function of the animal. The effects of endocrine disruption may not be apparent until an animal reaches reproductive age and developmental abnormalities impair its ability to produce or raise offspring. However, substances such as pituitary and thyroid hormones, which influence growth, development and behaviour, can also affect male sex hormones or other endocrine systems.