Parathyroid Hormone And Tetany: Regulation, Functions, And Clinical Signs

Parathyroid Hormone And Tetany: Regulation, Functions, And Clinical Signs

Give an account of synthesis, function and regulation of parathyroid hormone. Add a note on tetany. (or) Enumerate the actions of thyroxin.
Answer:

Synthesis of parathormone:

• Prepro-PTH containing 115 amino acids enters endoplasmic reticulum of cells of parathyroid glands.
• Prepro-PTH is converted into pro-PTH containing 96 amino acids
• Pro-PTH enters golgi apparatus
• Here, it is converted into PTH.

Functions:

1. On bones:

• PTH stimulates osteoclasts and osteoblast to cause resorption of calcium from bones.
• Thus, it increases plasma Ca2⁺ and decreases plasma PO₄^3-.

2. On kidneys:

1. Increases reabsorption of calcium from renal tubules.
2. Thus, decrease Ca2⁺excretion in urine and increases serum Ca2⁺ level.
3. PTH decrease reabsorption of PO₄^3- level
4. PTH increases the formation of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol.

3. On GIT:

• Increases calcium absorption from the gut.

4. On lactating mothers:

• Decreases the amount of Ca2⁺ secreted into the milk.

Regulation:

1. Directly by serum Ca2⁺ level.

Decrease in serum Ca2⁺ level.
↓
Stimulate parathyroid gland
↓
Increases PTH production
↓
Mobilization of Ca2⁺ from bones.
↓
Normalises serum Ca2⁺.

2. Indirectly by serum PO₄^3-

Increase in serum PO₄^3- level.
↓
Decreases serum
↓
Ca2⁺ level
↓
Inhibit 1,25-dihydroxycholecalciferol.
↓
Increases PTH.

Tetany:

• A low ionic Ca2⁺ level in the blood results in tetany.

Features:

• Tingling and numbness in the extremities.
• Feeling of stiffness with cramps in extremities
• Laryngeal stridor associated with airway obstruction.
• Chvostek’s sign – facial irritability resulting from ipsilateral contraction of the facial muscles.
• Carpopedal spasm – flexion at the wrist and thumb with hyperextension of remaining fingers.
• Troussaeu’s sign – In the lower limbs toes are plantar – flexed and feet are drawn up on inflation of a BP cuff.
• Hypotension, arrhythmia.
• Decreased permeability of the cell membrane.
• Seizures, mental retardation.
• Hair loss.
• Dry skin with brittle nails.