Chapter IV - Endocrine System
The parathyroid glands are small endocrine glands in the neck, usually located within the thyroid gland, which produce parathyroid hormone. Most often there are four parathyroid glands but have been known to number six or eight.
Parathyroid hormone is a small protein that takes part in the control of calcium and phosphorus homeostasis, as well as bone physiology. When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated to release hormone into the blood. It then stimulates osteoclasts to break down bone and release calcium into the blood. The sole purpose of the parathyroid glands is to regulate the calcium level in our bodies within a very narrow range so that the nervous and muscular systems can function properly.
The single major disease of parathyroid glands is overactivity of one or more of the parathyroid lobes, which make too much parathyroid hormone causing a potentially serious calcium imbalance. This is called hyperparathyroidism; it leads to hypercalcemia and osteitis fibrosa cystica.
The adrenal glands (also known as suprarenal glands or colloquially as kidney hats) are the triangle-shaped endocrine glands that sit atop the kidneys. They are chiefly responsible for regulating the stress response through the synthesis of corticosteroids and catecholamines, including cortisol and adrenaline.
Anatomically, the adrenal glands are located in the abdomen, situated on the anterosuperior aspect of the kidneys. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra and receive their blood supply from the adrenal arteries.
It is separated into two distinct structures, the adrenal medulla and the adrenal cortex, both of which receive regulatory input from the nervous system. As its name suggests, the adrenal medulla is at the center of the adrenal gland surrounded by the adrenal cortex.
The adrenal medulla is the body's main source of the catecholamine hormones epinephrine and norepinephrine.
The cortex produces a group of steroid hormones. These are glucocorticoids, mineralocorticoids, and gonadocorticoids.
1. Glucocorticoids: Glucocorticoids regulate the metabolism of carbohydrates, proteins, and fats. Various glucocorticoids include cortisol, cortisone, and corticosterone. Of these, cortisol has the greatest activity. Secretion of the glucocorticoid hormones is regulated by the adrenocorticotrophic hormone (ACTH) secreted from the anterior pituitary Glucocorticoids also regulate the concentration of glucose in the blood.
2. Mineralocorticoids: Aldosterone is one of the major mineralocorticosteroids in human beings. It regulates the electrolyte and water balance of the body. It maintains the homeostasis of sodium and potassium in the body It promotes reabsorption of sodium by the kidneys into the blood and excretion of potassium by the kidneys. It also promotes water conservation by reducing the urine output.
3. Gonadocorticoids: Gonadocorticoids are considered to stimulate the development of male secondary sexual characteristics like hoarseness of voice, presence of body and pubic hair, etc. Androstenedione and androstenolone (dehydro-3-epiandrosterone) are gonadocorticoids produced from the adrenal cortex.
· Hypoadrenalism (e.g. due to Addison's disease)
· Cushing's syndrome
· Congenital adrenal hyperplasia
· Conn's syndrome
· Pheochromocytoma is a catecholamine-secreting tumor of the adrenal medulla.
Cushing's syndrome or hypercortisolism is an endocrine disorder caused by excessive levels of the endogenous corticosteroid hormone cortisol. It may also be induced iatrogenically by treatment with exogenous corticosteroids for other medical conditions. When Cushing's is suspected, a dexamethasone suppression test (administration of dexamethasone and frequent determination of cortisol and ACTH levels) and 24-hour urinary measurement for cortisol have equal detection rates. A novel approach is sampling cortisol in saliva over 24 hours, which may be equally sensitive.
Cortisol is secreted by the adrenal glands under regulation by the pituitary gland and hypothalamus. Strictly, Cushing's syndrome refers to excess cortisol of any etiology. Cushing's disease refers only to hypercortisolism secondary to excess production of adrenocorticotropin (ACTH) from a pituitary gland adenoma.
A pheochromocytoma is a tumor of the medulla of the adrenal glands originating in the chromaffin cells, which secretes excessive amounts of catecholamines, usually epinephrine and norepinephrine. The diagnosis can be established by measuring catecholamine and metanephrines in plasma or urine. One diagnostic test used in the past for a pheochromocytoma is to administer clonidine (Catapres®), a centrally-acting alpha-2 agonist used to treat high blood pressure.
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Chapter IV - Endocrine System