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.