PARATHROMONE

Hypercalcemia may occur in the absence of osseous metastases in squamous cell carcinoma esophagus, probably from parathormone-related peptide secreted by tumor cells.

C i.e. Parathormone (PTH) promotes absorption of Ca²⁺ from intestine promotes absorption of calcium from intestineQ via activation of vitamin D (1,25, dihydroxy cholecalciferol)

• A large amount of calcium is filtered in the kidney but 98-99% of filtered Ca²⁺ is reabsorped: (d/t action of PTH) – 60% from proximal tubuleQ

– 25% from loop of henle

– Small extent in distal tubule

• Calcitonin is stimulated when there is hypercalcemiaQ.

C i.e. Increased Parathormone secretion

Ans. is ‘c’ i.e., Osteoporosis

o Several clinical trials have been performed using an exogenous PTH analogue (1-34hPTH; teriparatide) that has been approved for the treatment of established osteoporosis in both men and women.

o Exogenously administered PTH appears to have direct actions on osteoblast activity, with biochemical and histomorphometric evidence of de novo bone formation early in response to PTH, before activation of bone resorption.

o Subsequently, PTH activates bone remodeling but still appears to favor bone formation over bone resorption.

• The parathyroid glands are ovoid bodies measuring about 6 mm long in their greatest diameter.
• They are four in number and are closely related to the posterior border of the thyroid gland, lying within its fascial capsule.
• The two superior parathyroid glands are the more constant in position and lie at the level of the middle of the posterior border of the thyroid gland.
• The two inferior parathyroid glands usually lie close to the inferior poles of the thyroid gland. They may lie within the fascial sheath, embedded in the thyroid substance, or outside the fascial sheath

Ans. is ‘b i.e., Major regulating factor is parathormone

Calcium excretion by kidney

Calcium is both filtered and reabsorbed in kidneys but not secreted.

Thus renal excretion of kidney is –

Renal calcium excretion = Filtered calcium – Calcium reabsorbed

Normally, 99% of filtered calcium is reabsorbed in kidney.

1) PCT : Maximum calcium is reabsorbed in PCT. About 65% of filtered calcium is absorbed here.

2) Loop of Henle : About 25-30% of calcium is reabsorbed in loop of Henle (only from thick ascending limb).

3) DCT and collecting tubules : About 4-9% of calcium is reabsorbed in DCT and collecting tubules.

Primary controller of renal tubular reabsroption is parathormone. It increases calcium reabsorption in Loop of Henle (thick ascending limb) and distal tubules.

Vitamin D (calcitriol) and calcitonin also stimulate calcium absorption in thick ascending limb of loop of Henle and distal tubule, although these hormones are not as important quantitatively as PTH in reducing renal excretion.

Ans. is `b’ i.e., Decreased serum Ca⁺⁺

Ans. is ‘b’ i.e., Increases bone resorption

Paratharmone

• Paratharmone is 84 aminoacid polypeptide hormone secreted from parathyroid glands.
• Its secretion is regulated by plasma Ca++ concentration — fall in Ca++ concentration increases the secretion of paratharmone.
• Actions
• PTH increases resorption of calcium from bone → most prominant action.
• PTH increases calcium reabsorption from distal tubules of kidney.
• PTH decreases phophate reabsorption from proximal tubules of kidney.
• PTH has no direct effect on calcium absorption from intestine but increases it indirectly by enhancing the formation of calcitriol by activating loc-hydroxylose.
• PTH decreases calcium level in milk, saliva, and ocular lens — may be responsible for development of cataract in hypothyroidism.

Drugs related to paratharmone

 Cinacalcent

• It activates the Ca++ sensing receptor (CaR) in parathyroid glands and blocks PTH secretion.
• It is indicated in secondary hyperparathyroidism due to renal disease and in parathyroid tumor.

Teriparatide

• This is recombinant preparation of 1-34 residues of amino terminal of human PTH.
• It is used in osteoporosis.
• The effect is faster and more marked than that produced by estrogen and bisphosphonates.
• Teriparatide in the only agent which stimulates bone formation whereas estrogen and biphosphonate only check bone resorption.

Ans. is ‘b’ i.e., DCT

Paratharmone

• Paratharmone is 84 aminoacid polypeptide hormone secreted from parathyroid glands.
• Its secretion is regulated by plasma Ca++ concentration — fall in Ca++ concentration increases the secretion of paratharmone.

Actions

• PTH increases resorption of calcium from bone  → most prominant action.
• PTH increases calcium reabsorption from distal tubules of kidney.
• PTH decreases phophate reabsorption from proximal tubules of kidney.
• PTH has no direct effect on calcium absorption from intestine but increases it indirectly by enhancing the formation of calcitriol by activating la-hydroxylose.
• PTH decreases calcium level in milk, saliva, and ocular lens — may be responsible for development of cataract in hypothyroidism.

Ans. is ‘a’ i.e., Decreases phosphate level