parathyroid glands

parathyroid

 [par″ah-thi´roid]

1. near the thyroid gland.

2. a parathyroid gland.

3. a preparation containing parathyroid hormone from animal parathyroid glands; used for diagnosis and treatment of hypoparathyroidism.

parathyroid glands four small endocrine bodies in the region of the thyroid gland; they contain two types of cells: chief cells and oxyphils. Chief cells are the major source of parathyroid hormone (PTH), the secretion of which is dependent on the serum calcium level. Through a closed-loop feedback mechanism a low serum calcium level stimulates secretion of PTH; conversely, a high serum calcium level inhibits its secretion. The essential role of PTH is maintenance of a normal serum calcium level in association with vitamin D and calcitonin. It does this by exerting its effects on bone, kidney, and gastrointestinal tract. In bone, it enhances bone resorption by increasing digestion of the bone matrix by osteoclasts, which produces calcium that gets released into the bloodstream. In the kidney, PTH increases the excretion of phosphate and the reabsorption of filtered calcium. In the intestine, it increases intestinal absorption of calcium. The parathyroid glands may be subject to either hyperparathyroidism or hypoparathyroidism.

parathyroid glands

Four, yellow, bean-shaped bodies, each about 0.5 cm long, lying behind the THYROID GLAND, usually embedded in its capsule. The parathyroids secrete a hormone, parathyroid hormone (parathormone or PTH), into the blood if the level of calcium in the blood drops. This hormone promotes the release of calcium from the bones, controls loss in the urine and increases absorption from the intestine, thus correcting the deficiency in the blood. Maintenance of accurate levels of blood calcium is more important, physiologically, than the strength of the bones. Secretion of abnormal quantities of PTH from a parathyroid tumour can lead to bone softening. Underaction of the parathyroids causes a dangerous drop in the blood calcium.

Sandström,

Ivor V., swedish anatomist, 1852-1889.

Sandström bodies - Synonym(s): parathyroid glands

parathyroid glands

small clumps of endocrine tissue (usually four) on the back of the thyroid gland which secrete parathyroid homone (PTH). This acts to increase calcium ion concentration in the extracellular fluid, counterbalanced by calcitonin from the thyroid gland which has the opposite effects. Together they correct any changes in blood [Ca²⁺] by action on absorption of calcium from the gut, its deposition in bone and its excretion by the kidneys. See also hormones; Table 1.

Table 1: Hormones

Site of production	Name of hormone	Main targets	Involved in regulating:	Secretion controlled by:
Hypothalamus	Releasing and inhibiting hormones	Anterior pituitary (via local blood vessels)	Secretion of anterior pituitary hormones	Other brain regions; feedback re regulated hormones and their actions
	Neurohormones released from posterior pituitary:
	Oxytocin	Uterus, breasts	Labour and lactation	Afferent information from target organs
	Antidiuretic hormone (ADH, vasopressin)	Kidneys	Water loss: ECF volume and osmolality	Hypothalamic osmoreceptors
Anterior pituitary	(Human) growth hormone (H)GH	Most cells	Growth and metabolism	Hypothalamic releasing and inhibiting hormones via local blood vessels
	Prolactin	Breasts	Milk production
	Trophic hormones:
	Thyroid-stimulating (TSH)	Thyroid gland	Thyroid secretions
	Gonadotrophins	Ovary or testis	Germ cell maturation and hormone secretions
	Adrenocorticotrophic (ACTH)	Adrenal cortex	Cortisol secretion
Pineal body	Melatonin	Widespread, including brain, thymus, etc.	Sleep/wake cycle Antioxidant Immune system	Hypothalamus; varying light input from retina
Thyroid	Thyroxine Triiodothyronine Calcitonin	Most cells Bone, kidneys, gut	Cellular oxidative metabolism Decreases ECF [Ca2+]	TSH from anterior pituitary. Negative feedback from blood hormone concentration ECF [Ca2+]
Parathyroids	Parathormone	Bone, kidneys, gut	Calcium and phosphorus absorption, secretion and turnover in bone. Increases ECF [Ca2+]	ECF [Ca2+]
Adrenal: Cortex	Cortisol Aldosterone Androgens	Most cells Kidneys Gonads & other tissues	Metabolism Response to stress Na and K balance Sex characteristics and reproductive function	ACTH from anterior pituitary ECF [Na+] [K+] Renin-angiotensin ACTH
Medulla	Adrenaline Noradrenaline	Heart, smooth muscle, glands	Cardiovascular and metabolic adjustments to activity and stress	Sympathetic nervous system
Atrial wall	Atrial natriuretic hormone	Kidneys	Blood volume; increases sodium (therefore also water) loss in urine	Stretch of atrial wall by venous pressure
Gonads: Testis	Androgens (mainly testosterone)	Genitalia and other tissues	Reproductive function and sex characteristics	Anterior pituitary gonadotrophins
Ovary	Oestrogens Progesterone	Uterus, breasts and other tissues	Menstrual cycle, pregnancy, lactation
Pancreas	Insulin, glucagon Somatostatin	Most cells Other secretory cells in the pancreas	Blood levels, storage and cellular uptake of nutrients, notably glucose, but also proteins and fats	Blood levels of nutrients; autonomic nervous system; other gastrointesinal hormones
Alimentary tract
Stomach	Gastrin	Gastric acid-secreting cells	Gastrointestinal functions: motility, digestive juices and other secretions	Local chemical and mechanical factors in the alimentary tract
Small intestine	Secretin Cholecystokinin- pancreozymin (CCK-PZ) Somatostatin, motilin Other peptide hormones including vasoactive intestinal peptide (VIP)	Widespread on GI tract	Several GI functions including bile flow, pancreatic enzyme and exocrine secretions	Ingestion of food, distension of GI tract