Parathyroid Gland

Introduction: Parathyroid gland and hormone.

Parathyroid gland is essential for life.  It is a highly vascular and tiny gland measuring about 6 mm in length 3 mm in width and 2 mm in thickness its weight is only 120 mg it is present along with the thyroid gland.

 Parathyroid glands are 4 in numbers. Two on the left side and two on the right side. One parathyroid gland is present in the superior pole and one in the inferior pole of the thyroid gland on both sides.

 Location: In humans there are 4 parathyroid glands Tu to present in the superior for and to in the inferior pole of the thyroid gland. these glands are present on the posterior surface of the thyroid gland covered by internal and external layer of thyroid capsule.

 Two types of cells are present in parathyroid glands

 Oxyphil cells:  functions not clearly known but it is assumed that it helps the chief cells and may transform into Mein transform into cheap thrills chief sales chief chief sales.

 Chief cells: secret parathyroid hormone..

Parathyroid hormone

 biological Half Life less than 20 minutes it is a polypeptide and secreted in the blood vessels like other hormones.

 Functions of parathyroid hormone 

Main function is to regulate calcium concentration in the extracellular fluid and intracellular fluid. When calcium level is  low in the blood- it increases calcium level in the blood. Its main function is on bones, kidneys, and gastrointestinal tract.

 PTH receptors  couple to Z protein causing activation of of adenylyl cyclase which increases the formation of intracellular c AMP.

                                   Action of PTH (parathyroid hormone) 

Action of parathyroid hormone described under bones Kidneys gastrointestinal tract and lactating mammary glands.

On bones

The action of PTH on bone is prolonged, slow, and less sensitive prolonged.

It causes bone reabsorption that promotes osteolytic action and increases plasma calcium and decreases plasma phosphate. this action is achieved by the following mechanism

There are two types of cells in the bone responsible for maintenance of bone density. They are osteoblast which increases calcium and phosphate in the bone and increases bone formation. It is responsible for providing power to the bone.

Osteoclasts are present in the bone which is responsible for the reabsorption of the bone.

 Parathyroid hormone goes to the osteoblast where it gets attached with ‘parathyroid hormone receptors’ and stimulates the formation of Rank ligase osteoblast these triggers to rank receptors on the osteoclast.

 This rank receptor stimulates the osteoclast to form many types of chemicals ‘acid phosphatase’ and other proteolytic chemicals so that reabsorption of bone occurs. Calcium phosphate and other chemicals are released into the circulation thereby increasing calcium levels in circulation.

On  kidneys

 The action of PTH on kidneys is quick and sensitive for minor changes of serum calcium level. Increases calcium absorption in distal convoluted tubule by an active process so decreases calcium excretion in the urine.

Mechanism of action

 The distal convoluted tubule is impermeable to Sodium, Potassium, calcium, and other minerals so absorption of calcium is due to the presence of parathyroid hormone

 On the distal convoluted tubule, G receptors are present which combine with the PTH and stimulate adenyl cyclase which stimulates the formation of the cyclic AMP from ATP, and Protein kinase is formed which will cause transcription and translation that will form many calcium channels in the cell membrane of the distal convoluted tubule cells.

 The calcium ions enter into the distal convoluted tubule cells. The Sodium -Potassium channels in the distal convoluted tubule remove three sodium ions from the cells and two potassium into the cells. From another channel the sodium enters into the cell in exchange of calcium , through calcium sodium channel which is a secondary active transport system. Main active system is Sodium Potassium channel and energy produced by that is used in the calcium sodium channel. In this way calcium is absorbed in the distal convoluted tubule.

Posphate ions are secreted in the distal convulated tubule. This is Phosphaturic action of PTH.

On GIT

Phosphaturic action of PTH causes reduction of phosphate ion level in the blood, which increases the production of  1,25 dihydroxycholecalciferol the active form which is the active form of Vitamin D.

Mechanism of action

7, dehydrocholesterol is supplied by diet milk, egg, etc. Ultraviolet rays of the sun convert it into cholecalciferol. Cholecalciferol enters the liver where hydroxylation occurs by 25α-hydroxylase and forms 25 hydroxy cholecalciferol (25HCC) that will enter the kidneys.

In kidney’1,25α-hydroxylase’ hydroxylate ‘25 hydroxy cholecalciferol’ into  ‘1,25 dihydroxy cholecalciferol’, the active form of D3, also known as ‘Calcitriol’.

The calcitriol’ enters the enterocytes –lining cell of the small intestine and binds with the cytoplasm receptors. The receptor – calcitriol’ complex enters the nuceus,where it binds with the gene of DNA, and a new mRNA is synthesized –transcription occurs. The mRNA initiates new protein synthesis by translation. The newly formed protein is expressed on the surface cell membrane and opens several calcium channels so that more and more calcium ions are absorbed in the enterocytes than in the blood.

On the lactating mammary gland

Parathyroid hormone reduces calcium secretion in milk.

Regulation of PTH secretion

Decrease calcium ions in the blood stimulates PTH secretion , so that the level of calcium ions in the blood is maintained.

Increase serum phosphate level:

• decreases serum calcium level that will stimulate PTH secretion.
• Inhibits calcitriol formation that will stimulate PTH secretion.

Product of serum calcium x serum phosphate in plasma remains constant.Increase in the serum phosphate leads to  decrease in the serum calcium and vice versa.