They work in conjunction with the nervous system to control the homeostasis of the body, and to anticipate future events such as birth, lactation, fighting or fleeing. The hormone reaches many cells but, except for hormones affecting growth and some general metabolic processes, only certain target or end-organ cells respond. However, a hormone may sometimes inhibit a cell's activity, e.g., calcitonin inhibits osteoclasts' resorption of bone. In its usual concentrations, a hormone's action is called physiological . Pharmacological effects are seen when abnormally high quantities are injected.
Some peptide hormones have local paracrine , rather than distant, endocrine, effects.
The extent to which the injected fluid restores the animal to a normal condition gives a measure of its hormone content. The hormone is a product synthesized and released by glandular cells, mostly epithelial, but some are modified neurons or muscle cells. Cells have organelles associated with synthesis, e.g., granular or smooth ER, Golgi complex, and may store the hormone or prohormone as membrane-bound inclusion granules. Lysosomes may be used to destroy excess hormone. Actin filaments are used to discharge the granules by exocytosis .