Hormones - Mechanisms Of Action, The Hypothalamus, The Pituitary Gland, The Thyroid Gland, The Parathyroid Glands - Major hormones

Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream from where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. The word hormone comes from the Greek word, hormon, to stir up, and indeed excitation is characteristic of the adrenaline and the sex hormones. Most hormones produce an effect on specific target tissues that are sited at some distance from the gland secreting the hormone. Although small plasma concentrations of most hormones are always present, surges in secretion trigger specific responses at one or more targets. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.

The concentrations of several important biological building blocks such as amino acids are regulated by more than one hormones. For example, both calcitonin and parathyroid hormone (PTH) influence blood calcium levels directly, and other hormones affect calcium levels indirectly via other pathways.