The endocrine system—the other communication system in the body—is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to guide processes such as metabolism, growth, and sexual development. Hormones are also involved in regulating emotional life.
The thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive and cause drowsiness and a sluggish metabolism when the thyroid is underactive.
Near the thyroid are 4 tiny pea-shaped organs, the parathyroids, which secrete parathormone to control and balance the levels of calcium and phosphate in the blood and tissue fluids. This, in turn, affects the excitability of the nervous system.
The pineal gland is a pea-sized gland that apparently responds to exposure to light and regulates activity levels over the course of the day.
The pancreas lies in the curve of the duodenum and controls the level of sugar in the blood by secreting insulin and glucagon.
Brief history of endocrinology
The concept of neurosecretion was first elucidated by Ernst Scharrer and colleagues in the 1930s on the basis of the morphologic study of the hypothalamus of fish and mammals. Seventeenth-century English anatomist William Harvey, who questioned some of Galen’s conclusions, described the heart as a 4-chambered pump that moves blood through arteries and veins, not air.
In 1849, Berthold transplanted testes from normal cocks to capons and cocklike feathers reappeared. Addison recognized the relationship between low blood pressure, muscular weakness, weight loss, bronzing of skin, and the pathology of the suprarenal gland in 1855. In 1871, Hilton-Fagge related the cretinoid state to a congenital inadequacy of thyroid function in early childhood. Eight years later, Gull related dry skin, sparse hair, puffiness of the face and hands, and a swollen tongue to myxedema, the pathological deficiency of thyroid function in adults (goiter). In 1902, Balysis and Startling extracted and identified the first hormone secretin (secreted by cells in the intestinal mucosa), and, in 1927, McGee isolated and purified substances that were androgenic in small amounts (microgram levels). He used a bioassay to test his substance. He applied it to the beak of sparrows, and the beak darkened.
The endocrine system consists of endocrine glands that produce and secrete hormones into the blood stream to reach and act on target cells of specific organs. These hormones regulate the body’s growth, and are involved in cell to cell communication, control metabolic activity, sleep-wake homeostasis, and altered regulation or dysregulation of adaptive response in various physiologic and pathophysiologic states. The hormones are released into the bloodstream and may affect one or several organs throughout the body.
The concept of endocrine function thus was expanded to paracrine, autocrine, juxtacrine, and intracrine functions, whereas the classic endocrine system included the traditional endocrine axes. The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, suprarenals, 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. Our life exists through maintenance of a complex dynamic homeostasis or equilibrium, which changes constantly by intrinsic or extrinsic factors or stressors. Thus, stress is defined as threatened homeostasis that is established by physiologic and behavioral adaptive responses of the organism. 
The endocrine system is regulated by feedback in much the same way that a thermostat regulates the temperature in a room. For the hormones that are regulated by the pituitary gland, a signal is sent from the hypothalamus to the pituitary gland in the form of a “releasing hormone,” which stimulates the pituitary to secrete a “stimulating hormone” into circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.
The immune system is the third integrative system maintaining homeostasis. Endocrine and neural factors influence the immune response, and, in turn, cytokines—the secretions of lymphocytes, monocytes, and vascular elements—modulate both endocrine and neural functions. The immune system is a communication network that recognizes foreign antigens such as bacterial toxins and fungi and secretes signaling cytokines that regulate brain, endocrine, and immunocyte function. Virtually all endocrine changes involved in the adaptation of the stress, regulation of reproduction and homeostasis are integrated with specific behaviors. 
Pituitary gland lies in the base of the skull in a portion of sphenoid bone and consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis). The size of the gland, of which the anterior lobe consists of two thirds, varies considerably. It measures 13x9x6 mm and weighs approximately 100 mg. It may double in size during pregnancy.
Superiorly, the pituitary gland is covered by diaphragma sellae, or sellar diaphragm. The diaphragma sellae has a 5-mm wide central opening that is penetrated by the hypophyseal stalk. Embryologically, the pituitary gland originates from 2 distinct places. Rathke’s pouch, a diverticulum of the primitive oral cavity (ectoderm), gives rise to the adenohypophysis. The neurohypophysis originates in the neural ectoderm of the floor of the forebrain. Pituitary function is regulated by 3 interacting elements-hypothalamic neurosecretions, so-called releasing factors, feedback effects of circulating hormones, and autocrine and paracrine secretions of pituitary itself.