Each hormone attaches to a different set of receptors in the body.

Although a hormone can reach all cells in the body, only certain of those cells have receptors for that hormone. A hormone causes a reaction, such as a change in metabolism, in certain target cells that have the corresponding receptor.

Cells that lack that hormone's receptor are unaffected.

The endocrine system, one of the two main systems for communication and control in the animal body, is responsible for chemical signaling via hormones. The nervous system, a network of specialized cells (neurons) that transfer signals along designated routes, is the other primary communication and control system. These signals, in turn, govern neurons, muscles, and other organs.

The type of secreting cell and the path taken by the signal to its target distinguish the many modes of signaling between animal cells.

Endocrine signals, also known as hormones, are released into extracellular fluid by endocrine cells or ductless glands and travel to target cells via circulatory fluids. The binding of a hormone to a receptor specific for that hormone causes a biological reaction. Paracrine signals influence adjacent cells, whereas autocrine signals influence the secreting cell.

Neurotransmitters function locally as well, while neurohormones act across the body. Pheromones are chemicals that are released into the environment to aid communication between animals of the same species.

Cytokines and growth factors (polypeptides), prostaglandins (modified fatty acids), and nitric oxide are examples of local regulators that carry out paracrine and autocrine signaling (a gas).

Animal hormones are classified into three types: polypeptides, steroids, and amines. Hormones trigger various response pathways depending on whether they are water-soluble or lipid-soluble. Endocrine cells that release hormones are frequently found in glands that are dedicated in part or entirely to endocrine signaling.

Endocrine cells respond immediately to stimuli in a basic endocrine pathway. Through contrast, a sensory cell absorbs the input in a straightforward neuroendocrine route.

Hormone pathways can be controlled by either negative feedback, which dampens the stimulus, or positive feedback, which amplifies the stimulus and pushes the reaction to completion.

Molting and development in insects are regulated by three hormones: PTTH, an ecdysteroid whose release is induced by PTTH, and juvenile hormone. The sequence of developmental phases that leads to an adult form is brought about by the coordination of signals from the neurological and endocrine systems, as well as the regulation of one hormone activity by another.

The majority of anterior pituitary hormones are tropic hormones that control hormone production by acting on endocrine tissues or glands. TSH, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and adrenocorticotropic hormone (ACTH) are anterior pituitary tropic hormones (ACTH).

The effects of growth hormone (GH) are both tropic and nootropic. It directly promotes development, influences metabolism, and encourages the synthesis of growth factors by other tissues.

PTH, which is produced by the parathyroid glands, induces bone to release Ca2+ into the blood and promotes Ca2+ reabsorption in the kidneys. PTH also stimulates the kidneys to activate vitamin D, which increases the absorption of Ca2+ from meals in the intestine. Calcitonin, a thyroid hormone, has the opposite impact on bones and kidneys as PTH. Adults of certain animals, but not humans, require calcitonin for calcium homeostasis.

Neurosecretory cells in the adrenal medulla produce epinephrine and norepinephrine in reaction to stress, which mediates different fight-or-flight responses. The adrenal cortex secretes glucocorticoids like cortisol, which regulate glucose metabolism.