PARATHYROID GLAND & ITS HORMONES

A SIMPLE AND UNDERSTANDABLE PRESENTATION OF PARATHYROID GLAND PHYSIOLOGY.

1. PARATHYROID GLAND & HORMONES PRESENTED BY- Dr. FIRDOSH ROZY 1

2. INTRESTING FACT Richard Owen, (1804 – 1892) who discovered the parathyroid glands in 1850 in a rhinoceros 2

3. The endocrine system regulates body activities by releasing hormones (chemical messengers) into the bloodstream, where they are carried throughout the entire body. Glands secrete their products (hormones) into the extracellular space around the secretory cells.The secretions diffuse into capillaries and are carried throughout the body by the circulatory system. The specific cells which are affected by a hormone are called target cells. Circulating hormones may linger in the blood for minutes to hours or exerting their effects for a prolonged period of time 3

4. Prathyroid gland Parathyroids are two paired endocrine glands located on the thyroid glands. 4

5. DEVELOPMENT Endodermal proliferation of 3rd and 4th pharyngeal pouches. 5

6. ANATOMY Two pairs of glands 80 – 97% >4 – 13%, 3 – 3% Light yellowish to reddish brown Oval or lentiform shaped Bilobed 5%, multilobed 1% 5x3x2 mm 40 – 50 gms 6

7. Superior parathyroids – more consistent in location Subcapsular 1cm above the intersection between inferior thyroid artery and the recurrent laryngeal nerve 7

8. Inferior parathyroids variable in location inferior, posterior or lateral to the lower pole 61% Inferior to lower pole in close relation to the thyrothymic ligament 26% within cervical part of thymus 8

9. Position of parathyroid 9

10. 2% in the mediastinal portion of thymus 0.2% in the mediastinum In some cases they fail of descent – located above the position of superior parathyroid glands surrounded by remnants of thymic tissue Superior parathyroids lie posterior and inferior parathyroid lie anterior to recurrent laryngeal nerves POSITIONS OF PARATHYROID – 10

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12. HISTOLOGY 2 Tpes of cells- CHIEF CELLS OXIPHIL CELLS Histology 12

13. (Principal Cells): These cells release PTH and are more numerous, smaller, with a slightly eosinophilic cytoplasm CHIEF CELLS; OXIPIL CELLS; They are found individually, or clustered in groups. The function of these cells is unknown. 13

14. PHYSIOLOGY Major function of the parathyroid gland is homeostasis of calcium via Parathyroid hormone (PTH) PTH is synthesized in the parathyroid gland as a precursor hormone the pre proparathyroid hormone 115amino acid which is cleaved first to pro parathyroid hormone 90 and finally to the 84amino-acid 14

15. Half-life of 2 to 4 minutes Secretion mainly controlled by ionized calcium levels via calcium sensing receptors ( CaSR ) CaSR are G protein coupled receptors present on the chief cells and it is expressed on the surface of the parathyroid cell and senses fluctuations in the concentration of extracellular calcium Calcium binds on CaSR and reduces intracelular c AMP decreases the PTH secretion 15

16. PTH functions to regulate calcium levels via its actions on three target organs; bone, kidney, intestine. PTH secretion is also controlled by catecholamine levels and magnesium levels. It is stored in secretory vesicles within the cells, and released when required. 16

17. TARGET ORGANS FOR PTH Some effect on intestine also 17

18. The Role of the Parathyroid Glands PTH raises the blood calcium level by: 1. breaking down the BONE (where most of the body's calcium is stored) and causing calcium release. 2. increase the GIT ability to absorb calcium from food. 3. increasing the KIDNEY's ability to retain calcium that would otherwise be lost in the urine. 18

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21. PTH ACTION ON BONE FORMATION Increase osteoblastic number • Increase collagen synthesis • Increase alkaline phosphatase activity • Increase local growth factors: IGF and transforming factors 21

22. PTH ACTIONS ON KIDNEY Increase calcium and magnesium reabsorption from ascending loop and distal tubule • Increase chloride reabsorption from proximal tubule • Decrease phosphate,sodium and bicaronate reabsorption from the proximal tubule • Increase proximal tubule 1-alpha-hydroxalase activity 22

23. PTH ACTIONS ON INTESTINE Increases Ca absorbtion indirectly via 1,25- didroxcolecalciferol . 23

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25. CALCITONIN Polypeptide hormone • Synthesized and secreted by the parafollicular C-cells of the thyroid gland Action: receptors for Calcitonin are found in the bones and the kidneys. • It inhibits the activity of osteoclasts and also increase Ca++ in the urine 25

26. Vitamin D-3 formed in the skin when a cholesterol precursor, is exposed to ultraviolet light. Activation occurs when the substance undergoes 25-hydroxylation in the liver and 1-hydroxylation in the kidney. The primary action of 1,25-(OH)2 D3 is to promote gut absorption of calcium by stimulating formation of calcium-binding protein within the intestinal epithelial cells. VIT D 26

27. Vitamin D also promotes intestinal absorption of phosphate ion, although the exact mechanism is unclear. Negatively charged phosphate ion may passively flow through the intestinal cell because of flux of the positively charged calcium ion. In bone, vitamin D may play a synergistic role with parathyroid hormone ( PTH ) in stimulating osteoclast proliferation and bone resorption . Compared to parathyroid hormone ( PTH ), vitamin D exerts a much slower regulatory effect on calcium balance. 27

28. Why Calcium is so Important? A specific gland just to maintain Ca level ! It has a wide range of functions 28 Stimulate hormone secretion. Muscle contraction. Blood clotting. Nerve function- transmit nerve impulse. Necessary for the activation of some enzyme. Transport ion across membrane. Maintain regular heart beat (conduct electricity). Normal level of Ca in the blood is around 10mg /dl. ( Ganong’s review of med. Phy .)

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