cell that produced it.
             Two important terms are used to refer to molecules that bind to the hormone-binding sites of receptors:
             · Agonists are molecules that bind the receptor and induce all the post-receptor events that lead to a biologic effect. In other words, they act like the "normal" hormone, although perhaps more or less potently.
             Natural hormones are themselves agonists and, in many cases, more than one distinct hormone binds to the same receptor. For a given receptor, different agonists can have dramatically different potencies.
             · Antagonists are molecules that bind the receptor and block binding of the agonist, but fail to trigger intracellular signalling events.
             Antagonists are like certain types of bureaucrats - they don't themselves perform useful work, but block the activities of those that do have the capacity to contribute. Hormone antagonists are widely used as drugs.
             The concentration of hormone as seen by target cells is determined by three factors:
             · Rate of production: Synthesis and secretion of hormones are the most highly regulated aspect of endocrine control. Such control is mediated by positive and negative feedback circuits, as described below in more detail.
             · Rate of delivery: An example of this effect is blood flow to a target organ or group of target cells - high blood flow delivers more hormone than low blood flow.
             · Rate of degradation and elimination: Hormones, like all biomolecules, have characteristic rates of decay, and are metabolized and excreted from the body through several routes. Shutting off secretion of a hormone that has a very short halflife causes circulating hormone concentration to plummet, but if a hormone's biological halflife is long, effective concentrations persist for some time after secretion ceases.
             The pituitary gland, which is located in the center of the skull, just behind the bridge of the nose, is about the size of a ...