Parathyroid Gland and Related Hormones

Vitamin D and Calcitonin

Prof. K. Sivapalan

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Structure of Bone

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Cells in Bone

• Osteocytes- resting cells.• Osteoblasts are active in bone formation• Calcification occurs only in bone and not usually

in tissues.• Osteoclasts are responsible for removal of bone• Remodeling- continuous removal and formation.

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Bone structure .

• Collagen fiber laid down by fibroblasts is the basic inter cellular substance.

• Hydroxyappetite [Ca10(PO4)6(OH)2] crystals + other surface crystals, CaHPO4 are deposited on collagen by osteoblasts.

• Osteocytes are arranged in Havesian system.• Stimuli of bone formation-

– stress and strain.– growth factors and hormones.– (calcium and protein)

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Plasma Calcium.

• Calcium phosphate readily ionizes and equilibrates with plasma.

• Plasma calcium- – 50% bound to protein, – 5% complex with anions of plasma, – 45% ionized (active).

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Calcium Balance

• Gain during growth.• Plateau- middle age.• Loss in old age -

osteoporosis.

Vitamin D.

• Control of synthesis:• Feed back in kidney- higher plasma calcium

inhibiting [production of inactive 24,25 dihydroxycholycalciferol]

• Parathormaone stimulation production in kidney

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Actions of Vitamin D.• Increases absorption of Calcium in intestine and

kidney [Calbindins in intestine, kidney etc].• Increases osteoclasts in bone.• Increase plasma calcium.• Bone formation ? Indirect through plasma

calcium levels.• Differentiation of immune cells and keratinocytes

of skin.• Rickets is the result of deficiency in children.

Low plasma calcium leads to non calcification of epyphysis with excess growth of cartilage and week bone. Adults- osteomalasia.

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CALCITONIN.

• From para follicular cells of thyroid.• 32 amino acids, 3000 mw.• Effects-• Reduce osteiclastic activity• Increase osteoblastic activity.• Reduce formation of osteoblasts.• Control- calcium feedback.

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PARATHYROID HORMONE

• gland- 4 lobs, behind thyroid.

• 84AA, mw- 9500.• Half life- 20 min.

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Actions of Parathyroid Hormone

• Maintenance of plasma calcium.• Bone-

– increase osteoclastic activity.– reduces osteoblastic activity.

• Kidney-– increase conversion of Vitamin D.– increase reabsorption of calcium– decrease re-absorption of phosphates.

• Intestine-– increase calcium absorption- direct action is mild.

Main action is through increased Vitamin D.

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Control of Secretion of Parathyroid Hormone.

• Calcium negative feedback.• β adrenergic stimulation increases

secretion.

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Hyper Parathyroidism

• Causes- tumors. (in pregnancy and lactation hypertrophy is normal).

• Effect- increased plasma calcium.• Bone-

– increased osteoclastic activity.– easily broken bones, multiple fractures with

mild trauma.– X ray- eaten up bone appearence.– increased alkaline phosphatase

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Hypoparathyroidism

• Reduced plasma Calcium • Tetany.• Carpo-pedal spasm

(Trousseau's sign)• Facial muscle twitch on

tapping.- (Chvostek's sign)