Calcitonin is a 32-amino acid polypeptide hormone secreted by parafollicular cells in the thyroid gland. It plays an important role in regulating blood calcium levels by decreasing blood calcium through increasing calcium deposition in bones, reducing calcium resorption from bones, increasing calcium excretion by the kidneys, and decreasing calcium absorption in the intestines. Calcitonin also acts to decrease blood phosphate levels through similar mechanisms on bones and kidneys. High calcium levels in the blood stimulate calcitonin secretion through calcium receptors in parafollicular cells.

1. Calcitonin
DANISH HASSAN
LECTURER, UNIVERSITY OF SARGODHA

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

6.  Metabolism
 Calcitonin is degraded and excreted by liver and
kidney

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.