The parathyroid glands secrete parathyroid hormone (PTH) which regulates calcium and phosphate levels in the blood. PTH increases blood calcium levels by promoting calcium absorption in the gut and bones. It decreases blood phosphate levels by enhancing phosphate excretion by the kidneys. Abnormalities in PTH secretion can cause hypocalcemia and hypercalcemia. Hypoparathyroidism is caused by low PTH and results in low blood calcium and high phosphate. Hyperparathyroidism is caused by excessive PTH and results in high blood calcium.

1. Parathyroid Glands
Introduction
• The parathyroid glands are two superior and
inferior pairs of small endocrine glands lying
usually on the posterior border of the thyroid
gland within the thyroid capsule.
Shape And Size
• They are oval or lentiform in shape. The
parathyroid glands are very small glands
measuring about 6mm long,3mm wide and 2mm
thick with dark brown color.

2. Histology.
• The parathyroid gland contain two kinds of cells.
1- Chief Cells
• The chief cells are the secretary cells producing
parathyroid hormone (PTH),these cells have
golgi apparatus and endoplasmic reticulum.They
also contain glycogen whose level falls during
their activity.

3. 2- Oxyphil Cells
• The oxyphil cells appear after
puberty.These cells are larger and have a
granular cytoplasm.The function of the
oxyphil cell is not known.oxyphil cell
degenerated chief cells.However,these
cells may secrete parathormone during
pathological condition,parathyroid
adenoma.

4. Hormone of parathyroid gland
• The parathyroid glands secrete a hormone
known as parathormone(PTH).
Chemistry of PTH.
• PTH has been isolated in pure form.PTH is
protein in nature having 84 amino acids.Its
molecular weight is 9500.
• Half-Life and Plasma Level.
• PTH has half-life of 10 minutes.The normal
plasma level of PTH is about 1.5 to 5.5ng/dL.

5. Metabolism
• 60 to 70% of PTH is degraded by Kupffer cells of
liver by means of proteolysis.Degradation of
about 20-30% PTH occurs in kidneys and to a
lesser extent in other organs.
Synthesis of PTH
• PTH is synthesized from the precursor called
prepro-PTH containing 115 amino acids.

6. First,the prepro-PTH enters the endoplasmic
reticulum of chief cells of parathyroid
glands.There it is converted into a
prohormone called pro-PTH,which contain
96 amino acids.Pro-PTH enters the golgi
apparatus where it is converted into PTH.

7. Actions of PTH
• The main function of PTH is to maintain a
normal calcium level in blood and also to
regulate phosphorus conc.
• Increased PTH leads to increased blood
calcium level,but decreased phosphorus
level.
1- Effect on blood Ca++ and PO4.

8. • Increases the blood Ca++ Conc.due to
increased calcium absorption from bone
and decreased calcium excretion by
kidneys.
• Decreases the blood PO4 concentration
due to increased PO4 excretion by
kidneys.

9. 2- Effects on bone.
• PTH increases absorption of Ca++ from
bones in the following ways,
• Activation of all osteoclasts.
• Rapid formation of osteoclasts from
mesenchymal stem cells.
• Delay in conversion of osteocytes into
osteoclasts.

10. 3- Effects on Kidneys.
• Increased renal tubular reabsorption of
calcium.
• Increased reabsorption of Mg++ and H+
ions.
• Increased PO4 excretion.
• Decreased reabsorption of
phosphates,Na+,K+ and amino acids.

11. 4-Effects on GIT.
• PTH converts 25 hydroxychole calciferol into
1,25 dihydroxycholecalciferol which causes,
• Increased calcium and PO4 absorption from the
GIT.
5- Effects on lactating mammary
glands.
• Reduces Ca++ secretion in milk.

12. Regulation of PTH Secretion.
• Blood level of Calcium is the main factor
regulating the secretion of PTH.Blood
phosphate level also PTH secretion.
Blood Level of Calcium.
• PTH secretion is inversely proportional to
blood calcium level.Increase in blood
calcium level decreases PTH secretion.

13. The condition when PTH secretion decreases are
• Excess quantities of calcium in the diet.
• Increased vitamin D in the diet.
• Increased resorption of calciumfrom the bones.
Blood Level of Phosphate
• PTH secretion is directly proportional to blood
phosphate level.

14. • Whenever the blood level of phosphate
increases,it combines with ionized calcium
to form of calcium hydrogen phosphate.
This decreases ionized calcium level in
blood which stimulates PTH secretion.

15. Abnormalities of PTH Secretion
Hypoparathyroidism
• Decreased secretion of PTH is called
hypoparathyroidism.
• It leads to hypocalcemia.
Causes
• Surgical removal or destruction of the parathyroid gland
during thyroidectomy.
• Autoimmune disease.
• Deficiency of receptors for PTH in the target cells.
• Infections.

16. Clinical Features
• Hypocalcemia.
• Hyperphosphatemia.
• Difficulty in respiration.
Treatment
• Administration of calcium with vitamin-D.

17. Tetany
• Tetany is an abnormal condition characterized
by hyperexcitability of nerves and skeletal
muscles resulting in painful muscular
spasm(Involuntary contraction of muscle or
group of muscles) particularly in feet and hand.
The signs and symptoms of Tetany
1-Hyper-reflexia and Convulsions
• The increased neural excitability (Overactive
reflex action) and convulsive muscular
contractions.

18. 2-Carpopedal Spasm
• Carpopedal spasm is the spasm (violent and
painful muscular contraction) in feet and hand
that occurs due to hypocalcemia
3-Cardiovascular Changes
• Dilatation of heart.
• Hypotension.
• Heart failure.
• Prolonged duration of ST segment and QT
interval in ECG.

19. Other Features
• Decreased permeability of the cell membrane.
• Dry skin with brittle nails.
• Hair loss.
• When the calcium level falls below 4 mg/dL it
becomes fatal.During such sever hypocalcemic
condition,tetany occurs so quickly that a person
delelops spasm of different groups of muscles in
the body.

20. • Worst affected are the laryngeal and
bronchial muscles which develop
respiratory arrest resulting in death.
Treatment
• Administration of calcium with vitamin-D.

21. Hyperparathyroidism
• Hypersecretion of PTH is called
hypreparathyroidism.
• It results in hypercalcemia.
• Hyperparathyroidism is of three types.
1-Primary hyperparathyroidism.
• It is due to the development of tumor in one or
more parathyroid glands.Sometimes,tumor may
develop in all the four glands.

22. 2- Secondary Hyperparathyroidism
• It is due to the physiological compensatory
hypertrophy of parathyroid glands in response to
hypocalcemia which occurs due to other
pathological condition such as,
– Chronic renal failure.
– Vitamin D deficiency.
– Rickets.

23. Tertiary Hyperparathyroidism
• Hyperplasia (abnormal increase in the
number of cells) of all the parathyroid
glands that develops due to chronic
secondary hyperparathyroidism is called
tertiary hyperparathyroidism.