The endocrine system is a network of eight ductless glands that produce and release hormones that regulate homeostasis, metabolism, growth and development, tissue function, sexual function, reproduction, sleep and mood, among other things, into the blood. Unlike the nervous system, whose action helps the body react immediately to change, the endocrine system controls changes that happen to the body over a long period of time; from minutes, hours, to years of change. The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.
The hormones released by the endocrine system can be grouped into three classes based on their structure: steroids (cholesterol derivatives, secreted by the gonads, adrenal cortex, and placenta), peptides (small-chain amino acids, secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys), and amines (tyrosine derivatives, secreted from the thyroid and the adrenal medulla). These hormones affect the target cells by two different mechanisms. Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, and transduce their signal through second messenger systems into the target cell's interior. Steroid hormones, in contrast, pass through the plasma membrane of the target cells and act in a two step process in which they bind to nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex, in turn, binds to DNA and activates specific genes, increasing the production of proteins.
The endocrine system uses cycles and negative feedback which helps to control the delicate balance of hormones in the bloodstream and regulate physiological functions. However, minor changes in the function of one or more of the glands of the endocrine system, changes in the blood's fluid and electrolyte balance, or several external factors such as stress or infection can distort the balance of hormones and lead to an endocrine disorder, or endocrine disease. Hypothyroidism, thyroid cancer, fertility issues, diabetes, and other metabolic disorders are typical examples of problems that originate from imbalances of the enodcrine system.