2. Calcitonin
Secreted by the parafollicular cells or clear cells
(C cells), situated amongst the follicles in thyroid
gland.
In lower animals, the parafollicular cells.
4. Chemistry and Synthesis
Calcitonin is a polypeptide chain with 32 amino
acids. Its molecular weight is about 3,400.
It is synthesized from procalcitonin
5. Plasma Level and Half-life
Plasma level of calcitonin is 1 to 2 ng/dL.
It has a halflife of 5 to 10 minutes
7. Actions Of Calcitonin
Plays an important role in controlling the blood calcium
level.
It decreases the blood calcium level and thereby
counteracts parathormone.
Calcitonin reduces the blood calcium level by acting
on:
Bones
Kidneys
Intestine
8. On bones
Calcitonin stimulates osteoblastic activity and
facilitates the deposition of calcium on bones.
At the same time, it suppresses the activity of
osteoclasts and inhibits the resorption of calcium
from bones.
It inhibits even the development of new osteoclasts
in bones
9. On kidney
Increases excretion of calcium through urine, by
inhibiting the reabsorption from the renal tubules
10. On intestine
Prevents the absorption of calcium from intestine
into the blood.
11. On Blood Phosphate Level
With respect to calcium, calcitonin is an antagonist
to PTH.
But it has similar actions of PTH, with respect to
phosphate.
It decreases the blood level of phosphate by acting
on:
1. Bones
2. Kidneys
12. On bones
Inhibits the resorption of phosphate from bone and
stimulates the deposition of phosphate on bones.
13. On kidney
Calcitonin increases the excretion of phosphate
through urine, by inhibiting the reabsorption from
renal tubules
14. Regulation Of Calcitonin Secretion
High calcium content in plasma stimulates the
calcitonin secretion through a calcium receptor in
parafollicular cells.
Gastrin also is known to stimulate the release of
calcitonin.