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Major Endocrine Glands and Hormones

Now that we understand the basic ideas about what hormones are and how they work, we can list the major endocrine glands, the hormones they produce, and the effects they can cause in the body.
  • Pituitary gland and hypothalamus: The pituitary gland is located just below the hypothalamus, at the base of the brain. The hypothalamus-pituitary complex is a major point of interface and coordination between the endocrine and nervous systems. As noted before, the hypothalamus, in addition to functioning in the central nervous system, produces many hormones. Most of these are hormones that control the secretions of the anterior and posterior pituitary gland. Two hormones produced by the hypothalamus are stored and ultimately released by the posterior pituitary.
  • Antidiuretic hormone (ADH), sometimes referred to as vasopressin, regulates the ability of the kidneys to reabsorb water, and plays a major role in the body’s overall water balance.
  • Oxytocin functions mainly in females to stimulate the release of milk from mammary glands after childbirth.
Six major hormones are generated and secreted by the anterior lobe of the pituitary.
  • growth hormone (GH), sometimes called somatotropin, stimulates cells to grow in size and number by causing certain metabolic changes.
  • prolactin (PRL) causes changes mainly in females, promoting the growth of breast tissue and the secretion of milk after childbirth.
The other four hormones produced by the pituitary gland function in regulating the action of other endocrine glands, and are therefore often called tropic hormones.
  • Thyroid-stimulating hormone (TSH) stimulates the thyroid gland.
  • Adrenocorticotropic hormone (ACTH) controls the production of the hormones of the adrenal cortex.
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) influence the regulation and hormone production of the primary reproductive organs in both males and females.
  • Thyroid gland: Thyroxine (T4) is the major hormone secreted by the thyroid gland, located in the neck. T4 consists of an amino acid complexed with the mineral iodine, and acts to increase the rate of metabolism (respiration) of all body cells. The thyroid gland also produces triiodothyronine (T3), which functions similarly to T4, and calcitonin, which regulates the blood levels of the minerals calcium and phosphorus, which are important for healthy bone formation.
  • Parathyroid glands: Located on the surface of the thyroid gland, these tiny structures produce only one hormone, parathyroid hormone (PTH), which is sometimes referred to as parathormone. PTH functions in conjunction with calcitonin and vitamin D to precisely regulate the levels of calcium, and less importantly phosphorus, in the blood and various body regions. Since calcium is involved in a wide variety of vital functions (nerve and muscle transmission, bone formation, and blood clotting), its regulation is clearly important for the normal functioning of the body.
  • Adrenal glands: The adrenal glands are located on the surface of the kidneys, and are subdivided into two parts, the adrenal cortex and the adrenal medulla, which are actually distinct glands that secrete different hormones. The adrenal cortex produces two major hormones, aldosterone and cortisol, both steroids. Aldosterone plays a major role in the regulation of sodium and potassium levels, vital in nerve transmission, via the action of the kidneys. Cortisol raises the level of glucose in the blood by stimulating gluconeogenic pathways (especially the production of glucose from proteins and fats) during periods of stress, and apparently plays an additional role in the regulation of the inflammatory and immune responses. The adrenal medulla secretes two hormones, epinephrine (often called adrenalin) and norepinephrine (noradrenalin). These hormones are released in conjunction with sympathetic nervous stimulation, and play a role in increasing heart and respiration rates, while additionally raising the blood glucose level, in preparation for potential intense activity.
  • Pancreas: The pancreas is the largest gland in the body. In the vicinity of the stomach and attached to the small intestine by connective tissue, it has exocrine as well as endocrine functions. Its exocrine function consists of its ability to produce the major digestive enzymes and release them, through the pancreatic duct, into the duodenum of the small intestine. The endocrine portion of the gland, which is comprised of cell clusters called islets of Langerhans, produces and secretes two major hormones, insulin and glucagon. Both are intimately involved with the maintenance of a relatively constant level of glucose in the blood. Insulin lowers blood glucose concentrations by allowing cells to take up and utilize glucose for fuel, while glucagon has the opposite effect. It reacts to low blood sugar levels by promoting the breakdown of glycogen in the liver and its release as glucose into the blood. Disorders involving these hormones include diabetes and hypoglycemia.
  • Ovaries and testes: These are the primary reproductive organs in females and males, respectively, and in addition to their gamete-producing activities, they also secrete steroid hormones. In females, the ovaries secrete estrogen and progesterone, which control sexual development, the menstrual cycle, and certain aspects of pregnancy. In males, the testes produce testosterone, which stimulates sperm production, controls sexual development, and is responsible for the male secondary sexual characteristics (growth of facial hair, etc.).
  • Other endocrine activity: The pineal gland secretes melatonin, which may play a role in the normal sleep cycle as well as helping to regulate the female reproductive cycle. The thymus gland secretes thymosin, which helps lymphocytes to mature. Digestive organs, the kidneys, and the heart are all capable of secreting hormones with varying effects.

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