* It is secreted by parathyroid gland.
* Parathyroid gland has two types of cells.
* Chief cells and oxyphil cells.
* Chief cells secrete parathyroid hormone.
* Generally oxyphil cells dont produce para thyroid hormone but they produce when needed.
SYNTHESIS OF PARATHYROID HORMONE :
* Parathyroid hormone is synthesized as Pre Pro hormone which is converted to Pro hormone then to parathyroid hormone.
PHYSIOLOGICAL ACTIONS OF PARATHYROID HORMONE :
* Parathyroid hormone generally increases level of ionized calcium in the blood.
* It acts on bone , kidney , and GIT.
ACTION OF PARATHYROID HORMONE ON KIDNEY :
* Receptors for parathyroid hormone are present on basolateral membrane of proximal convoluted tubular cells.
* When parathyroid hormone binds to its receptor , it activate protein kinase- a.
* Protein kinase-a inactivates Sodium Potassium cotransporter present on basolateral membrane of PCT cells.
* Parathyroid hormone blocks the reabsorption of phosphate at proximal convoluted tubule.
* Which leads to excretion of phosphate in urine.
* Parathyroid hormone also stimulate formation of 1, 25 dihydroxy cholecalciferol which is active form of Vitamin D in PCT cells.
* Active form of Vitamin D increases reabsorption of calcium from kidney and intestine .
ACTIONS OF PARATHYROID HORMONE ON BONE :
* Parathyroid hormone receptors are present on osteoblasts which eventually activates osteoclasts.
* Activity of osteoclast on bone :
* When osteoclasts are activated they help in resorption of mineralized bone.
* Which results in release of calcium , phosphate and hydroxyproline into the blood from the bone.
* This calcium phosphate when reaches kidney , calcium is reabsorbed and phosphate is released into urine.
* In this way ionised calcium levels in the blood increases in the presence of parathyroid hormone.
EFFECT OF PARATHYROID HORMONE ON GIT :
* Parathyroid hormone acts on proximal convoluted tubular cells and increases synthesis of active form of Vitamin D.
* Active form of Vitamin D increases the reabsorption of calcium from diet in GIT.
* It even increases phosphate reabsorption in the intestinal cells.
REGULATION OF PARATHYROID HORMONE :
* It is mainly regulated by plasma calcium level.
* When plasma calcium level increases , parathyroid hormone secretion decreases.
* When plasma calcium level decreases , parathyroid hormone secretion increases.
* Chief cells have calcium sensing receptors on their cell membrane.
* These receptors sense the presence of ionized calcium in the blood and eventually changes parathyroid hormone secretion.
* Plasma phosphate levels :
* When plasma phosphate levels increases , parathyroid hormone levels also increases which eventually leads to decrease in phosphate level in blood.
APPLIED PHYSIOLOGY OF PARATHYROID HORMONE :
* HYPERPARATHYROIDISM :
It is mainly of two types :
1) Primary hyperparathyroidism
2) Secondary hyperparathyroidism
PRIMARY HYPER PARATHYROIDISM :
* It occurs due to to tumour present within the parathyroid gland.
* Clinical features include :
* Excess of parathyroid hormone in the blood.
" Excessive ionized calcium in the blood.
* Excess of parathyroid hormone leads to demineralization of bone and bones become weak and fibrotic , this condition is called " osteitis fibrosa ".
* There is excessive phosphate , calcium and hydroxyproline in urine.
SECONDARY HYPER PARATHYROIDISM :
* It occurs secondary to chronic renal diseases and chronically low levels of serum calcium .
* Chronically low levels of ionized calcium stimulate excess of parathyroid hormone release.
Which results into hypertrophy of parathyroid glands.
HYPO PARATHYROIDISM :
* This condition is very rare.
* Causes of hypoparathyroidism includes :
* Congenital hypoparathyroidism or
DIGEORGE SYNDROME :
* In the syndrome congenitally pharyngeal pouches mainly 3 and 4 don't develop well.
* So parathyroid gland and thymus won't be developed well.
* Which eventually leads to low parathyroid hormone and deficiency of cell mediated immunity.
FAMILIAL HYPO PARATHYROIDISM :
* In this condition subject have generally low parathyroid hormone due to low activity of parathyroid glands and reduced function of adrenal glands .
* Clinical features of hypo parathyroidism include :
* Decrease parathyroid hormone level.
* Less serum ionized calcium.
* Hypercalciuria.
* High serum phosphate level.
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